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http://www.archive.org/details/physiographyforhOOarey 




Yosemite Falls from Sentinel Hotel. 

A flood plain in the foreground. 

By permission of Oliver Lippincott. 

Frontispiece. 



PH YSIOGR A PHY 

FOR HIGH SCHOOLS 



BY 

ALBERT L. AREY, C.E. 

girls' high school 

FRANK L. BRYANT, B.S. 

ERASMUS HALL HIGH SCHOOL 

WILLIAM W. CLENDENIN, M.S., M.A. 

WADLEIGH HIGH SCHOOL 
AND 

WILLIAM T. MORREY, A.M. 

BUSHWICK HIGH SCHOOL 

NEW YORK CITY 



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D. C. HEATH & CO., PUBLISHERS 

BOSTON NEW YORK CHICAGO 



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Copyright, 191 1, 
By D. C. Heath & Co. 



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©CIA297068 



PREFACE 

For a number of years the authors have felt the need of a text 
that presents physiography from the high school point of view, 
both in content and in treatment. Since more than nine tenths 
of the pupils who enter secondary schools complete their formal 
education in such schools, the needs of this great class cannot be 
neglected ; and subjects must be planned with the fact in mind 
that secondary pupils should know the scientific explanation of 
the common phenomena of nature. 

A course in physiography in a college is naturally limited by the 
existence of parallel courses in astronomy, geology, and meteor- 
ology ; and the fear of overlapping has led college men to omit 
many valuable topics from their courses and from the texts which 
they have prepared. No such limitation is found in the high 
school, and teachers are at liberty to select such topics as will 
contribute most to the culture of the pupil. 

High school pupils should know of the earth as a whole, its 
relation to the other heavenly bodies, and the influence of its size, 
shape, and motions upon our daily life. They should know of 
the sun and the moon and their influence, and lack of influence, 
upon us. We have, therefore, included in our course, such astro- 
nomical topics as are necessary to this end. The pupils should 
also know of the natural resources of our country and their impor- 
tance, and should understand the influence of climate and physical 
environment upon a given region as well as upon the history and 
the development of our nation and of the civilizations of the world. 
We have, therefore, included topics usually treated only in geology, 
meteorology, and history. 

The abstract discussion of processes, as processes, belongs to 
the college rather than to the High School ; we have, therefore, 
discussed such topics as diastrophism, erosion, and the like in 
connection with concrete instances of their work. - 



IV PREFACE 

We are not in accord with those who would make physiography 
in the high school a regional subject. The secondary student 
easily masters a scientific treatment of a general topic such as 
mountains when the mountains of various regions are studied 
and contrasted ; but he fails to do so if mountains are discussed 
in a fragmentary manner in connection with descriptions of various 
regions. 

The treatment of the subject here presented has been in success- 
ful use in our class rooms many years, and we believe that it will 
be found equally satisfactory to others. 

This text contains more matter than can be mastered by first- 
year pupils, and we have indicated by smaller type the paragraphs 
that it is our custom to omit with first-year classes ; it is expected, 
however, that each teacher will make his own selection of the 
topics to be omitted. Where the subject is taught in the fourth 
year, no omissions will be necessary. 

The italicized words in the text are intended as a guide to the 
teacher and the pupil. Technical terms are italicized when first 
introduced, and are to be defined by the pupil. Sentences in 
italics are to be memorized. In descriptions of functions and 
properties, the important words are italicized for emphasis. This 
use of italics makes printed questions on the text unnecessary. 

The questions found at the end of each chapter cannot be 
answered by quoting the text. Each requires the pupil to draw 
an inference from some fact or facts stated in the text, or to exer- 
cise his judgment in contrasting facts. Some of them call upon 
him to exercise his imagination, and here suggestions from the 
teacher may be in order. We have found them particularly valu- 
able as a stimulant to independent thought on the part of the pupil. 



CONTENTS 

PART I 
THE EARTH AS A PLANET 

CHAPTER PAGE 

I. The Earth in Space 3 

II. Latitude, Longitude, and Time. 17 

III. The Moon 30 

IV. The Solar System 36 

V. Map Projection 50 

VI. Terrestrial Magnetism 58 

PART II 
THE AIR 

VII. Properties and Functions of the Air 65 

VIII. Temperature of the Air 73 

IX. Weight and Density of the Air 91 

X. Movements of the Air 99 

XL Moisture of the Air 117 

XII. Light and Electricity of the Air 129 

XIII. Weather and Climate 139 

XIV. Climate of the United States 160 

PART III 
THE SEA 

XV. General Characteristics of the Sea 183 

XVI. Movements of the Sea . 195 



vi CONTENTS 

PART IV 
THE LAND 

CHAPTER PAGE 

XVII. The Mantle Rock 219 

XVIII. The Bed Rock 246 

XIX. The Ground Water 270 

XX. The Work of Rivers 284 

XXI. Life History of a River . 310 

XXII. Lakes, Falls, and Rapids 315 

XXIII. Glaciers . 328 

XXIV. Plains and Plateaus 351 

XXV. Mountains 376 

XXVI. Volcanoes and Earthquakes 402 

XXVII. Shore Lines and Harbors 423 



PART I 

THE EARTH AS A PLANET 



PHYSIOGRAPHY 

CHAPTER I 
THE EARTH IN SPACE 

The earth is a ball nearly 25,000 miles around. It is composed 
of rock, with about three-fourths of its surface covered with 
oceanic waters having an average depth of only 2^ miles. The 
whole is surrounded by an envelope of air probably more than 
200 miles thick. 

On its surface we are unconscious of any motion because of the 
steadiness and freedom from jar, still we know that the familiar 
phenomena of the rising and the setting of the sun are due to 
the turning of the earth on its axis. At night the star dome 
appears to revolve about the earth, and gives us further evidence 
that we live on a ball that is turning in space uniformly and 
regularly. 

We also learn that while the earth is whirling, it is rushing 
through space with inconceivable velocity. While the seconds' 
pendulum of a clock makes one swing, the earth moves 18)^ miles, 
which is a thousand times the speed of the fastest express trains. 

In going this distance each second, the earth curves about one- 
ninth of an inch from a straight line. This slight rate curvature 
continued for a year, brings the earth around to the place of start- 
ing. 

The absolute uniformity of turning of the earth on its axis, 
and the regularity of movement as a whole about the sun, are 
of great service to mankind. The former affords a convenient 
means of measuring the length of the day, and the latter marks 
off the year. 



4 PHYSIOGRAPHY 

The earth is only one member of a family of rotating balls. 
This family, together with other bodies, is controlled by the 
sun and constitutes a system. 

A conception of the earth in space, as a member of the solar 
system, and some knowledge of conditions on other worlds, may 
give us clearer views of our real insignificance in space, time, and 
matter. 

Condition of the Interior of the Earth. — The interior of the earth is 
believed to be solid throughout, although the temperature is undoubt- 
edly above the melting point of materials on the earth's surface. The 
melting point increases with the pressure, but it is believed that the 
pressure raises the melting point faster than temperature rises as the 
center is approached, so that the fusion point is never reached. Be- 
cause the earth as a whole has a greater density than the material near 
the surface, the central portion is thought to be more dense than the 
so-called crust. 

Careful studies made of the variation in the position of the earth's 
axis, the effects of tide-producing forces acting upon the earth, and the 
velocity of earthquake waves through the earth, have led to the conclu- 
sion that the earth is more rigid than steel. 

Form, Size, and Weight. — Water and mud fly off from the fast 
rotating wheels of wagons and automobiles, when running on wet, 
muddy roads. This is due to the tendency of bodies to move in a 
straight line. When a body moves in a curved path it appears to 
be pulling away from the axis of rotation. This pull is called 
centrifugal force. The centrifugal force is greater, the greater the 
distance from the axis. 

Because of the rotation of the earth, the excess of centrifugal 
force developed in equatorial regions causes it to bulge out there 
and to flatten in the polar regions. The resulting form is that of 
an oblate spheroid. 

This does not necessarily indicate a one-time molten condition 
of the earth. The sea bulges at the equator and flattens at the 
poles, and the land wears down to sea level. 

The axis of rotation of the earth, or the polar diameter, is 
7,899.76 miles, and the equatorial diameter is 7,926.60 miles, the 
latter being nearly 27 miles more than the former. The ends of 



THE EARTH IN SPACE 5 

the earth's axis are called poles. The average of the different 
diameters of the earth is nearly 8,000 miles, and the circumference 
is about 25,000 miles. 

The surface area of the earth is nearly 197,000,000 square miles, 
of which 54,000,000 square miles are land. The earth is 5.6 times 
as heavy as a sphere of water the same size. 

Problem of Eratosthenes. — The first successful attempt to measure the 
size of the earth was made about 200 B.C. by Eratosthenes, an astronomer 
and geographer of Alexandria, Egypt. He learned that at Syene, the 




~suri% noon raw 
onJuneZlit 



Fig. 1. — Method of Eratosthenes 

A, a vertical pillar at Syene, is 23)2° north of the equator at E, a point on the Tropic of 
Cancer. B, a vertical pillar at Alexandria is 7. 2° or 5,000 stadia further north. C is the 
center of the earth and CA and CB radii. 



most southern city of Ancient Egypt, the gnomon or vertical pillar cast 
no shadow at noon on June 21st. At Alexandria, 5,000 stadia directly 
north of Syene, the sun's noon ray on the same day made an angle 
of 7.2 degrees with a vertical pillar. Assuming the earth to be a 
sphere, this angle of 7.2 degrees is equal to the angle formed at the center 
of the earth between radii to Alexandria and Syene. It follows that as 
these two places are on the same meridian, an arc of 7.2 degrees equals in 
length 5,000 stadia, so that 360 degrees, or the distance around the earth, 
equals 250,000 stadia. As the distance between Syene and Alexandria 
is about 500 miles, the circumference of the earth would be 25,000 miles, 
which is not far from the truth. 



6 PHYSIOGRAPHY 

EVIDENCES THAT THE SURFACE OF THE EARTH IS CURVED 

i. During every eclipse of the moon, that portion of the shadow 
of the earth cast on the moon always has a curved edge. 

2. The circumnavigation of the earth proves that it is not flat. 
It does not prove that the earth has the form of a sphere. It 
might, for instance, have the oval form of a football. 

3. As ships sail away their hulls disappear first, and as they 
come into port their masts appear first. This shows that the 
water surface is actually rounded up between us and the distant 
ship. 

4. At sea, the circle known as the horizon seems both to sink 
and to increase in size with an increase in elevation above the 
surface of the water. If the water surface were an extended 
plane, our area of visibility would not increase with an increase 
of elevation. 

5. That the weight of a body is about the same everywhere on the 
earth's surface shows that the earth is globular in form. The slight 
increase in weight noticed as one approaches the poles is in part due to a 
flattening of the earth's surface in those regions. 

6. That the apparent shifting of the sky position of the stars is directly 
proportioned to the distance traveled north or south, shows that the 
earth's surface is curved along north-south lines like a sphere. 

7. Places not on the same meridian have different times of day as a 
result of the curved shape of the earth's surface along east-west lines. 
If the earth were flat all places would have the same time. 

8. On the shores of a calm lake, away from the tides and swells, the 

-Wile — > ' « — niiie- 



—£ curvature of the Earth's surface 

Fig. 2. — Post Method for Measuring the Curvature of the Earth 

curvature of the earth may be measured by erecting in a straight line 
three posts, A, B, C, at the same height above the surface of the water. 
When looking with a telescope from the top of post A to the top of 
post C, the top of post B will be above the line of sight. If the distance 
from A to B is a mile, top of post B will be 8 inches above the line of 
sight, or the curvature of the water surface is 8 inches to the mile. In 
two miles the curvature is 8 inches X 2 squared, or 32 inches; in three 



THE EARTH IN SPACE 7 

miles, 8 inches X 3 squared, or 72 inches — that is, the curvature for any 
distance is equal to 8 inches multiplied by the square of the number of 
miles. 

THE MOVEMENTS OF THE EARTH 

Rotation and Revolution. — The earth has two principal motions, 
a uniform spinning motion called rotation, and a forward move- 
ment in its path about the sun called revolution. 

The angular rate of motion due to rotation is 15 an hour; 
and the absolute rate of motion of a particle on the earth's 
surface is greatest at the equator. Here it is 25,000 miles a day, 
or more than a thousand miles an hour, and decreases toward the 
poles, where it is nothing. The rate of motion of the earth as a 
whole, due to revolution, is about a degree a day. This amounts 
to 1,600.000 miles a day. 

ROTATION AND ITS EFFECTS 

i. The side of the earth that at any moment is turned toward 
the sun is in sunshine, and the side turned away from the sun is 
in darkness. The rotation of the earth from west to east pro- 
duces a movement of illumination and shadow around the earth 
from east to west. 

2. The eastern horizon is really sinking and the western horizon 
rising, which has the effect in the lower latitudes of making the 
sun, moon and stars appear to rise along the eastern and set along 
the western horizon. 

3. The period of rotation, in respect to the sun, is 24 hours, 
and determines the length of day. 

4. The slight bulging of the earth in the equatorial regions is due 
to rotation. 

5. Winds and ocean currents, because of the earth's rotation, are 
deflected to the right in the northern hemisphere and to the left in the 
southern hemisphere. This may be illustrated by pouring water upon 
a rotating globe. 

6. Every particle of matter on the earth's surface describes each day 
a circle. These circles are largest at the equator, and particles there 
consequently have the greatest velocity. This motion has the effect 



8 PHYSIOGRAPHY 

of lessening the weight of bodies, due to the tendency of bodies to fly- 
away from the center of rotation. Bodies therefore, for this reason, 
weigh less at the equator than in higher latitudes. 




Fig. 3. — Star Trails, Due to the Earth's Rotation, Made on a 
Photographic Plate. 

7. Bodies falling from a considerable height fall to the east of a ver- 
tical line suspended from the point of starting. 

Foucault Pendulum Experiment. — In 1851 Foucault, a French physi- 
cist, devised a remarkable proof of the earth's rotation by means of a 
pendulum. From the dome of the Pantheon in Paris he hung a heavy 
iron ball about a foot in diameter by a steel wire more than 200 feet long. 
The pendulum was set in motion and the plane of vibration seemed to 
rotate slowly toward the right. It can be easily shown by a simple 



THE EARTH IN SPACE 




Fig. 4. — Foucault's Experiment 



experiment that the plane of vibration of a pendulum remains fixed. 
The true interpretation must then be that the floor of the Pantheon was 
actually turning under the plane in 
which the pendulum was swinging. 

If the pendulum were suspended 
at the pole, the earth would turn 
around under it in twenty-four hours. 
The time required for the earth to 
shift entirely around under the plane 
of the vibrating pendulum increases 
as the latitude decreases. At the 
equator there would be no tendency 
for the earth to shift. 

REVOLUTION AND ITS EFFECTS 

Stars Shift Westward.— The 
movement of the earth, in its path 
about the sun, causes the sun to 
appear to move eastward among 
the stars. This has the effect of 
making the stars appear to shift westward about a degree a day. 
The real path the earth travels each year is called its orbit. 

The path which the sun appears to follow around the heaven once a 
year, as a result of the annual movement of the earth in its orbit about 
the sun, is called the ecliptic. It is so called because all the eclipses of 
the sun and moon occur when the moon is in the plane of this path. 

The zodiac is the belt of the heavens, 16 degrees wide, 8 degrees on 
each side of the ecliptic. It is so called because the constellation or 
groups of stars in it are thought to resemble or outline the forms of 
animals. The 360 degrees of the zodiac are divided into twelve equal 
parts, each called a sign. 

The Latin names, with the symbols used to represent them, are as 
follows : 

The Spring Signs The Autumn Signs 

f Aries, the Ram. — Libra, the Balance. 

8 Taurus, the Bull. W Scorpio, the Scorpion. 

n Gemini, the Twins. * Sagittarius, the Archer. 

The Summer Signs The Winter Signs 

2> Cancer, the Crab. V3 Capricornus, the Goat, 

ft Leo, the Lion. & Aquarius, the Waterman. 

^ Virgo, the Virgin. * Pisces, the Fishes. 



lo PHYSIOGRAPHY 

Change of Seasons. — As the earth moves forward around the 
sun, its axis is always tipped 23 ^ degrees from a perpendicular 
to the plane of its orbit. The earth's axis is always inclined in 
the same direction, so that during a revolution the axis remains 
parallel to itself in all positions. It is because of the (1) inclination 
of the earth's axis and its maintenance of (2) parallelism during a 
complete (3) revolution, that the change of seasons occurs. 

Cause of Unequal Days and Nights. — The same causes produce 
a shifting of the daily sky path of the sun during the year, and the 
consequent variations in length of daylight and darkness. 

Ellipse, Perihelion, and Aphelion. — The orbit of the earth has 
the form of an ellipse, with the sun at the north focus. The earth 
is at perihelion, or nearest to the sun, on January 2, when it 
is 91,500,000 miles away, and at aphelion, or farthest from the 
sun, on July 3, when it is 94,500,000 miles away. 

In the sketch (Fig. 5) the earth is shown in four positions as 
it makes its annual journey about the sun. 

On December 21 the north pole is tipped away from the sun 
and in the middle of the long period of darkness. The noon tan- 
gent rays just reach the Arctic Circle, thus causing the whole 
area within that circle to be in darkness. At this time the sun's 
noon ray is vertical over the Tropic of Capricorn. It is winter 
in the northern hemisphere and summer in the southern. The 
area within the Antarctic Circle is lighted and the south pole is 
in the middle of the long period of sunlight. The days are shorter 
than the nights in the northern hemisphere. 

On March 21 the noon ray is vertical over the equator and 
the rays are tangent at the poles. Day and night are equal all 
over the earth. 

On June 21 the north pole is tipped toward the sun. The 
tangent noon rays just reach the Antarctic Circle, thus causing 
the area within that circle to be in darkness. At this time the 
sun's noon ray is vertical over the Tropic of Cancer. It is summer 
in the northern hemisphere and winter in the southern. The area 
within the Arctic Circle is lighted and the north pole is in the mid- 



THE EARTH IN SPACE 



II 




Fig. s — Four Positions of the Earth Corresponding to the Four Seasons 

In the winter and summer positions two different views are shown, one looking along a line 

vertical to the earth's orbit and the other looking parallel to the earth s orbit. 



12 



PHYSIOGRAPHY 



die of its long period of sunlight. The days are longer than the 
nights in the northern hemisphere.* 

Direction of Sunrise and Sunset. — The sun rises directly in the 
east and sets in the west only twice a year, on March 21 and 

Mjd-tfay Sit/, 
11 zemth 




Nadir , 

Fig. 6. — Showing Position of Sun's Apparent Daily Sky Paths at the Equator 
Paths are vertical to the horizon and days and nights always equal. 



September 23. At these two dates, called the Equinoxes, the 
sun's noon ray is vertical at the equator, or as more commonly ex- 
pressed, "the sun is crossing the line," and days and nights are 
everywhere equal. 

From the March, or Vernal Equinox, to the September, or Au- 
tumnal Equinox, in the northern hemisphere the sun rises north 
of east and sets north of west, and the days are longer than the 
nights. 

From the September to the March Equinox, in the northern 
hemisphere the sun rises south of east and sets south of west, 

*See Chapter VII for more exact length of days in higher latitudes at different times of year. 



THE EARTH IN SPACE 13 

and the nights are longer than the days. (Make corresponding 
statements for the southern hemisphere.) 

The northern journey of the sun culminates on June 21, 
called the Summer Solstice. The southern journey culminates on 
December 21, called the Winter Solstice. 



zenith 




Nadir 

Fig. 7. — Showing Position of Sun's Apparent Daily Sky Paths at Latitude 41 N. 
The paths are tipped toward the south, showing our long days in summer and our short days 
in winter. The direction of sunrise and sunset at different times of year may be read from 
the figure. 



The Sun's Daily Sky Path. — Because of the inclined position of 
the earth's axis, the sun's daily sky path (not only the sun's vertical 
ray and the sunrise and sunset position, but also each corresponding 
position for every moment of the day), migrates northward for 
one half of the year and then southward for the other half of the 
year. The effect of this is to bring about the regular changes in 
the inequality of the lengths of day and night. From a study of 
the above sketches, the shifting position of the sun's daily sky path 
for the year may be seen in places of different latitudes. The 
middle position is the sun's daily path for the March and Sep- 
tember equinoxes, and the position farthest north is the sun's 



14 



PHYSIOGRAPHY 



path for the June solstice, and the position farthest south is the 
sun's daily path for the December solstice. 
The planes of all the sun paths are always inclined from a 

Zjeni_rh_ 




Nadir 



Fig. 8. — Showing Position of Son's Apparent Daily Sky Paths at the Arctic Circle, 

Latitude 66^° N. 

On June 21 the sun remains above the horizon and on Dec. 21 below the horizon for 

the entire 24 hours. 

vertical an amount equal to the latitude of the observer, for the' 
planes are parallel to each other. See Figs. 6, 7, 8, and 9. 

QUESTIONS 

1. How do we know that the earth is whirling uniformly in space? 

2. Make a sketch of an oblate spheroid and draw in the axis and the 
equatorial diameters. Properly letter the sketch and locate the poles. 
Where is the centrifugal force due to rotation the greatest? The least? 

3. What is the area of the water surface of the earth? What substan- 
ces are as much as 5.6 times as heavy as water? 

4. Why are not the so-called evidences proofs that the surface of the 
earth is curved? Which evidences are strongest? Which weakest? 

5. What are the relative positions of daylight and darkness upon the 
earth? In what direction do they travel? 

6. Try to picture in your mind, by using a globe, the actual path 
which a particle at the equator describes due to the combined motion 



THE EARTH IN SPACE 



IS 



of rotation and revolution. Make a free-hand sketch to show the 
motion and describe it. 

7. A degree of longitude in latitude 40 equals about 53 miles. How 

Zenith 

Tie I t>o> 




Nadir 



Fig o. — Showing Position of Sun's Apparent Sky Paths at the North Pole 
Paths are nearly horizontal. The year is divided into two periods of sunlight and darkness. 



many miles an hour does a point in that latitude move? How does this 
result compare with the distance the earth as a whole moves in an hour, 
due to revolution? 

8. When riding on a railroad train, in what direction does the outside 
view from the window appear to move? What does this illustrate in 
respect to the apparent motion of the sun, moon and stars? 

9. What effect has centrifugal force, due to rotation, upon the weight 
of bodies at the surface of the earth? Where is this effect greatest? 
Where the least? 

10. The stars that rise and set, rise about four minutes earlier each 
night. Why? In a month these stars appear to shift westward. How 
far? What effect has this in twelve months? What is the real cause 
of the apparent shifting of the star dome? 

11. If the earth's axis were perpendicular to the plane of its orbit, 
would revolution cause a change of season? Suppose the inclination of 
the earth's axis varied during a single revolution, what effect would that 
have upon the change of seasons? Is revolution necessary for a change 
of seasons? Explain. 



16 PHYSIOGRAPHY 

12. In what direction does the sun's daily path through the sky shift 
from December 21 to June 21? During this period of six months, which 
is growing in length, our period of illumination or the period of darkness? 
Why? How is it from June 21 to December 21? 

13. Why is not the sun always the same distance from the earth? 

14. In about what latitude is the noon ray of the sun vertical on Jan- 
uary 1st? March 1st? July 1st? September 1st? At these different 
dates, which is the longer here, the daytime or the night? In what 
latitude approximately are the northern and the southern limits of 
illumination at these dates? 

1 5. State whether at these different dates the sun rises north or south 
of east and sets north or south of west. 

16. Why are the Tropics placed where they are? 

17. What is meant by the expression "sun crossing the line"? How 
often does this occur? In what direction is the sun migrating at each 
time? 



CHAPTER II 

LATITUDE, LONGITUDE, AND TIME 

LATITUDE 

The equator is the circle extending around the earth midway 
between the poles. Circles parallel to the equator are called 
parallels. The planes of all parallels, as well as the plane of the 
equator, are at right angles to the earth's axis. The distance ex- 
pressed in degrees, north or south of the equator, is called the latitude 
of a place. 

The axis of the earth extended northward marks the position 
of the north pole of the heavens. The elevation of the celestial or 
sky pole above the equator equals the latitude of the observer. 
The angle between a vertical line and the plane of the earth's 
equator also equals the latitude of the observer. 

Because the equatorial bulge makes the curvature of the sur- 
face of the earth grow gradually less from the equator toward 
the poles, degrees of latitude increase slightly in length toward the 
poles. Less curvature of the earth's surface in the higher latitudes 
means that the surface has the form of an arc of a larger circle. A 
degree, or 1-360 of the length of the circumference of a larger circle, 
is evidently longer than a degree of a smaller circle. A degree of 
latitude at any place is therefore 1-360 of the circle whose curvature 
is that of the meridian at that place. 

The circle N E S E' represents a meridian section of the earth, 
N S being the axis and E E' the equator. H H' is the plane of the 
horizon with the observer at O. 

O N' extends north and is parallel to the axis N S. The point Z 
is the zenith directly over the observer. 



i8 



PHYSIOGRAPHY 



The angle C E' is the latitude of the observer and equal to 
N' O H, the altitude of the north pole of the sky. 

Proof. — Angle Z N', the zenith distance of the north pole of the 
sky plus the angle HON' equals a right angle, or 90 degrees, since Z O 
is the perpendicular to H H'. 

Angle N C plus angle E' C equal a right angle, or 90 degrees, 
since the axis of the earth is perpendicular to the plane of the equator. 




Fig. 10.— The Altitude of the North Sky Pole, Angle N' O E, Equals the 
Latitude of Observer 0, Angle C E' 



Angles N C and N' Z are equal, being corresponding angles made 
by a line crossing two parallel lines. 

Therefore the complementary angle E' C O, the latitude of the ob- 
server, equals HON, the altitude of the north pole of the heavens. 

How to Find a North and South Line. — By the following meth- 
ods, a north and south line may be located: 

(a) On any clear night the direction of Polaris, when it is 
directly above or below the sky pole, is due north. This occurs 
twice in every twenty-four hours, when Polaris and Mizar, the 
star in the bend of the handle of the Big Dipper, are in a vertical 
line. 

(b) The direction of a magnetic needle, when corrected for 
variation, will enable one to locate a north and south line. 

(c) The direction of the shortest shadow cast on a horizontal 
plane by a vertical post is north and south. When the sun is at 



LATITUDE, LONGITUDE, AND TIME 



19 



its highest point in the sky, shadows are shortest. This occurs 
at solar noon, which is approximately noon, local time. 

Latitude Determined by Night. — The latitude of an observer may 
be found on any clear night by means of the Pole Star (Polaris). 




Fig. 11. — Showing the Rotation of the Heavens about the North Star 



The number of degrees of a heavenly body above the horizon is 
called its altitude. At the equator the North Star appears on the 
horizon, and its altitude is consequently zero. At 40 degrees north 
of the equator, for instance, the North Star is 40 degrees above the 
horizon (altitude 40 ); and at the north pole of the earth it is in 
the zenith (altitude 90 ). The altitude of the North Star in the 
northern hemisphere equals, therefore, the latitude* of the place 



20 PHYSIOGRAPHY 

where the observation is made. This is not always absolutely 
correct, since Polaris describes daily a circle, i3^° from the north 
pole of the sky. 

Latitude Determined by Day. — Another method of finding the 
latitude of a place is to measure the distance of the noon sun from 
the observer's zenith. At the time of the equinoxes the sun is on 
the sky equator, and the zenith distance of the noon sun from the 
zenith equals the latitude of the place where the observation is 
made. 

To find the latitude of a place at other times of the year by means of 
the zenith distance of the noon sun, certain corrections should be made. 
The Nautical Almanac gives the position of the noon sun in reference 
to the sky equator. This is called the sun's declination. In the north- 
ern hemisphere, if the sun is north of the sky equator, the zenith 
distance of the noon sun will be that number of degrees less than the 
latitude. If the sun is south of the sky equator, the zenith distance of 
the noon sun will be just that number of degrees more than the latitude 
of the place. 

The zenith distance of the sun should be found just as it crosses the 
observer's meridian, that is, when it is on a north and south line. 

LONGITUDE 

The lines that pass from pole to pole on the earth's surface 
are called meridians. Meridians are farthest apart at the equator 
and converge toward each pole. 

The meridian that passes through Greenwich, England, is the 
Prime Meridian, and the meridian from which longitude from o° 
degrees to 180 east and 180 west is reckoned. 

Definition, Prime Meridian, Use. — Longitude is the distance 
expressed in degrees east or west from the prime meridian. A 
degree of longitude at any place is 1-360 of the parallel of that place. 

The location of a place anywhere on the earth's surface may be 
found by determining its latitude and longitude. A place in lat- 
itude 40 degrees north and longitude 75 degrees west is on the 
parallel 40 degrees north of the equator, at a point where the 
meridian 75 degrees west of Greenwich crosses it. 



LATITUDE, LONGITUDE, AND TIME 21 

How Longitude is Determined. — Longitude is determined by 
finding the amount by which the noon at Greenwich is earlier or 
later than the observer's noon. Since the earth turns eastward 
through 360 degrees in 24 hours, it turns 15 degrees an hour, or 
1 degree in four minutes. An hour slower than Greenwich means 
that the place is 15 degrees west longitude, and an hour faster 
means that the place is 15 degrees east longitude. 

Various methods for the determination of longitude are used: 

(a) By the Chronometer, which is an accurate clock that keeps 
Greenwich time. Chronometer time is compared with local time 
found by taking an observation of the noon sun. At sea observa- 
tion is made with a sextant. Before noon the sun's altitude is 
increasing. When it ceases to increase the sun is on the meridian 
and the time is apparent noon. 

(b) By making a direct telegraphic comparison between the clock 
set to local time of the observer and that of some station of known 
longitude. The difference in time will give the difference in longi- 
tude between the two places. 

TIME 

How Time is Determined. — The rotation of the earth furnishes 
us with a measure of time. The day is a universal unit of time. 
It is the interval between two successive passages across a given 
meridian of a given heavenly body. If the sun is the heavenly 
body taken for reference, the day is called a solar day, if the moon 
a lunar day, and if a star a sidereal day. 

The three kinds of days may be better understood from a study 
of Fig. 12. E represents the earth in its orbit about the sun 5, and 
E' is the position of the earth a day later. M represents the moon 
in its orbit about the earth, and M' its position a day later. Far to the 
left of the diagram is a certain star so far away that lines drawn to it 
from any point on the earth's orbit are practically parallel. The moon 
M, the sun S, and a star S' are on the meridian with the observer at O. 

The earth rotates as it moves forward in its orbit. The direction of 
the motion of revolution of both earth and moon, and the direction of 
the motion of the rotation of the earth, when seen from above the north 
pole, are counter-clockwise, as indicated by arrows in figure. 



22 



PHYSIOGRAPHY 



The real movement of the earth of approximately a degree a day 
in its path or orbit about the sun causes the sun to appear to move 
among the stars eastward about a degree a day. This has the effect 
of making the stars rise four minutes earlier and set four minutes 
earlier on successive nights. In a year's time the stars come back to 
the same position in the sky at the same time of day, for four minutes 
each day of the 365 days of the year make about one whole day. 




Fig. i 2- -Diagram Showing Effect of Revolution of the Earth upon the 
Lengths of Different Kinds of Days 

Sidereal Day. — During one complete rotation of 360 degrees, the earth 
moves from its position at E, Fig. 12, to a new position at E', and the 
observer at is brought to 0'. The same star has again come to the 
observer's meridian and one sidereal day has ended. 

A sidereal day may then be defined as the interval of time between the 
passage of a star across a meridian and its next passage across the same 
meridian. It is divided into 24 sidereal hours. Astronomical clocks 
keep sidereal time and mark the hours from o to 24. The sidereal day, 
being about four minutes shorter than the sidereal noon, comes four 
minutes earlier each day, so that during a year it occurs at all hours 
of the day and night. 

Solar Day. — Since the forward motion of the earth in its orbit is about 
a degree a day, the earth must rotate eastward one degree more than 
360 degrees to bring the sun again to the observer's meridian, that is, 
the earth turns through 361 degrees from 0, Fig. 12, to 0" to complete 
one solar day. 

Lunar Day. — The daily motion eastward of the moon in its orbit is 
about 13 degrees. The earth must rotate 13 degrees more than 360 



LATITUDE, LONGITUDE, AND TIME 



23 




degrees to bring the moon again to the observer's meridian, that is, the 
earth turns through 373 degrees from O to O'", Fig. 12, to complete one 
lunar day. 

Mean Solar Time. — The apparent motion of the sun being faster when 
nearer the earth and slower when farther away, makes the sun a poor 
timekeeper. 

By taking the average length of all apparent solar days in a year, a 
definite length of our day is obtained. Our clocks and watches are regu- 
lated to keep this mean solar time. The apparent solar time read on 
the sun dial, and the mean solar 
time read from our clocks, agree 
only four times a year. This 
average day is called the mean 
solar day, and may be considered 
as being regulated by an imaginary 
sun that has a uniform motion and 
consequently crosses the meridian at 
regular intervals. 

The attempt to construct clocks 
with compensating devices that 
would keep real solar time was 
made during the eighteenth cen- 
tury. The variation in the sun's apparent motion was so complex, that 
apparent time clocks were abandoned early in the nineteenth century. 

The sun dial consists of two essential parts, a style or gnomon and a 
dial. The style is placed parallel to the earth's axis and casts a shadow 
on the dial. The different hours of the day are marked on the dial, and 
the shadow of the style cast by the sun passing over it, as the sun moves 
through the sky, indicates the time of day. 

The style is usually a rod or edge of a thin plate of metal, and 
being parallel to the earth's axis makes an angle with the horizontal 
dial-plate equal to the latitude of the place where the sun dial is 
located. 

Equation of Time. — When the sun does not cross the meridian until 
after mean noon time the sun is said to be slow, and when it crosses the 
meridian before mean noon the sun is said to be fast. The amount 
that the real sun is ahead or behind the imaginary average sun is called 
the equation of lime. 

The Civil Day. — Our ordinary day, called the civil day, begins 
at midnight and ends on the following midnight. Business is gen- 
erally suspended at that time, and the change of date can be made 



Fig. 13. — Son Dial 



24 PHYSIOGRAPHY 

then with the least confusion. The first 12 hours are called a.m. 
(ante-meridian), and the second period of 12 hours p.m. (post- 
meridian); 12 m. means noon or sun on the meridian. To find the 
exact time at which the sun is actually on the meridian the table 
for the equation of time must be consulted or an observation 
must be made. 

For a person who travels around the earth, the number of times 
the sun crosses his meridian would be one less if going westward 
and one more if going eastward, than it would be if he stayed at 
home. It is evident, then, that if the traveler does not add a day 
when going westward and drop a day when going eastward, upon 
his return his reckoning will differ one day from that at home. It 
has been agreed among mariners to make the change of date at 
the 1 80th meridian from Greenwich. 

To avoid confusion of dates on islands crossed by the meridian, 
an off-set eastward a few degrees is made about New Zealand and 
an off-set westward is made about the Fiji Islands. Another off- 
set eastward, is made to avoid passing across the extreme eastern 
part of Siberia. After passing through Bering Strait the date line 
returns to the 180th meridian. 

International Date Line. — The 180th meridian, together with 
the off-sets mentioned, constitute the international date line. The 
date on the western side of this line being a day later than on the 
eastern side, ships, in crossing it, omit a day in their reckoning 
when going westward, and repeat a day when going eastward. 

The Conventional Day. — The day which by international consent 
it has been decided that any country has at any moment, is called 
the conventional day. The conventional day begins at the inter- 
national date line, and moves westward 15 degrees an hour with 
the sun. Parts of two different days are on the earth at the same 
time. The midnight line, which is just opposite the noon sun, 
marks the forward or westward boundary of each advancing day. 

Local Time. — The mean solar time of any place is called its 
local time. Places of different longitude differ in local time four 
minutes for each degree. In going around the earth at the equa- 



LATITUDE, LONGITUDE, AND TIME 25 

tor, a distance of about 25,000 miles, the local time changes at 
the rate of one hour for a distance of about 1,038 miles. In lati- 
tude 40 degrees, a distance of about 801 miles, makes a difference 
of one hour in local time, and in latitude 60 degrees, 519 miles. 

Standard Time Belts in the United States. — Because of the con- 
fusion that resulted from each place keeping its own local time, 
especially along railroads extending east and west, most railroad 
towns readily gave up their own and adopted the time in use by 
the railroad. The number of railroads increased until at certain 
centers there were many railroads entering the same city, each 
with a different local time in use. Much confusion arose from 
having different local times used in the same place. A definite 
system of keeping time in the United States was decided upon, 
and in 1883 the different railroad lines put it into operation. 
This system is called Standard Time, and may be defined as the 
time based upon a certain meridian that is adopted as the time 
meridian for a definite belt of country. Its advantage is that 
neighboring places keep the same time, instead of differing a few 
minutes or seconds according to their longitude. This is of 
especial importance in the operation of railroads and telegraphs, 
and with the transaction of any business concerned with contracts 
involving definite time limits. The standard time meridians of the 
United States, as adopted, are 75 degrees, 90 degrees, 105 degrees, 
and 120 degrees west from Greenwich. 

This system has been extended to the remote possessions of the 
United States, and has spread over the greater portion of the 
world. 

Eastern Standard Time. — The mean solar time of the 75th 
meridian is used for places on both sides of that meridian and in a 
belt approximately 15 degrees wide, and is called Eastern Standard 
Time. This meridian runs through Philadelphia, and there local 
and standard time are the same. The time within this belt is five 
hours slower than Greenwich time. The so-called time belts have 
very irregular eastern and western boundaries, depending upon 
the location of cities upon the railroads. Study carefully Fig. 14 
and trace the time belt boundary lines. 



26 



PHYSIOGRAPHY 




LATITUDE, LONGITUDE, AND TIME 27 

Central Standard Time. — The time of the next belt westward is 
the mean solar time of the 90th meridian, called Central Standard 
Time, and is one hour slower than Eastern time. When it is noon, 
Eastern time, at Washington, Baltimore, Philadelphia, New York, 
and Boston, it is n a.m., Central time, at Chicago, Minneapolis, 
St. Louis, and New Orleans. 

Mountain Standard Time. — The next time belt westward uses 
the mean solar time of the 105th meridian, called Mountain Stan- 
dard Time. Denver, Colorado, is on this meridian, so that clocks 
in that city indicate both mountain and mean solar time. 

Pacific Standard Time.— The time belt on the extreme west of 
the United States covers the States on or near the Pacific coast, 
and has the mean solar time of the 120th meridian, called Pacific 
Standard Time. Time in this belt is three hours slower than in the 
eastern belt, and eight hours slower than Greenwich time. In 
Alaska, standard time is nine hours slower than Greenwich time. 

El Paso, Texas, has the peculiar condition of having four differ- 
ent systems of time in use. The mountain standard time belt 
tapers southward to a point at El Paso. This allows the Eastern, 
Mountain and Pacific time belts to meet. The standard time 
for Mexico, on the south, is 24 minutes later than Mountain time. 
The railroads that enter El Paso from the east, south and west 
bring their own time. Mountain time is used by the city officials 
of El Paso. 

Time Signals. — The time sendee of the United States is under 
control of the Government. By cooperation of the telegraph 
companies, time signals are sent out daily at noon, Eastern time, 
from the Naval Observatory at Washington, D. C, to nearly 
every telegraph station in the country. These regulate automati- 
cally more than 30,000 clocks, and drop time balls in scores of 
different ports of the Atlantic, Pacific, Gulf of Mexico, and Great 
Lakes coasts. Time signals for the extreme western part of the 
United States are distributed from Mare Island 'Navy Yard, in 
California. 



28 PHYSIOGRAPHY 



THE CALENDAR 

The very early calendar, worked out by the Romans, was based largely 
on the motions of the moon. As the yearly number of revolutions of the 
moon varies, the seasons and festivals did not keep in place, and the 
Roman calendar fell into a state of great confusion. The year consisted 
of ten months, March being the first and December the tenth and last. 
January and February were added later. There were about 29J/2 days 
in a lunar month, so the months were given 29 and 30 days alternately, 
beginning with January. The number of days in a week was probably 
based upon the number of planets then known, including the sun and 
moon. In the year 46 B.C., the Roman calendar was reformed by 
Julius Caesar, under the advice of Egyptian astronomers. 

The Julian Calendar. — The Julian calendar was planned without 
reference to the moon. It made three consecutive years of 365 days 
each, and the fourth of 366 days. The extra day was added to February, 
that month then having only 29 days, and the other months having 
alternately 30 and 31 days. The length of the Julian year was 365.25 
days, and since the true year has 365.24 days, the Julian year was .01 
of a day, or 11.2 minutes too long. 

This difference of 11. 2 minutes between the length of the Julian year 
and the year now in use amounts to a little more than three days in 400 
years. As a consequence, the date of the vernal equinox came continu- 
ally earlier in the Julian year. In 1582 the vernal equinox occurred on 
the nth of March. 

The Gregorian Calendar. — In that year Pope Gregory XIII directed 
that ten days be stricken from the calendar, so that March equinox 
might occur on March 21. A further reform was introduced at this 
time in order to prevent a similar occurrence. The Pope decreed that 
the centurial year should not be counted as a leap year except when 
divisible by 400. Thus 1800, 2100, and so forth, are not leap years, 
but 1600, 2000, and 2400 are leap years. 

The Gregorian calendar is now used in all civilized countries except 
Greece and Russia, where the Julian calendar is still in force in spite of 
repeated efforts to abolish it. The 14th of every month here is the first 
of the month there. 

In England it was adopted in 1752. Dates of events occurring before 
the Gregorian calendar was adopted are termed Old Style (0. S.), and 
those after the adoption New Style (N. S.). 

In order to gratify the vanity of Augustus Caesar, the month now 
bearing his name, formerly called Sextilis, was given 31 days so as to 
have as many as July, formerly called Quintilis, which was named for 



LATITUDE, LONGITUDE, AND TIME 29 

Julius Caesar. A day was accordingly taken from February, leaving 
only 28 days for that month, and given to August. Because of the 
superstition of having three months of 31 days each, together, September 
and November were reduced to 30 days, and October and December were 
given 31. 

QUESTIONS 

1. How may ships be located at sea? If city streets extend east and 
west and at right angles to avenues, how may places be located thereby? 
Compare the plan of locating a place in the city with that of locating the 
ship at sea. 

2. How may the following be determined in the southern hemi- 
sphere: — (a) Latitude by night? (b) Latitude by day? (c) A north and 
south line? 

3. At what time of day is longitude usually determined? Why? 

4. What is the circumference of the earth at the 60th parallel, as 
compared with the circumference at the equator ? 

5. Why is a solar day about four minutes longer than a sidereal day? 
Do solar days differ in length? Why? 

6. In laying out a north and south line by means of the noon sun, 
what besides a watch would be necessary? 

7. What are some of the practical advantages of having the civil day 
change at midnight? State any difference you may see between the 
civil day and the conventional day. 

8. How long has every day been on the earth before it reaches you? 
At what time by the clock at your place does a new day start on the 
earth? If Sunday is just east of the international line, what day is just 
west of the line? Explain. 

9. By how much does the local time of your place differ from stan- 
dard time? Why are the boundaries of the standard time belts so irregu- 
lar? 

10. At what hour do the noon time signals from Washington reach 
Chicago? Denver? Explain. 

n. What advantages has the sun over the moon for calendar pur- 
poses? State the reason for the present rule for leap year. 



CHAPTER III 
THE MOON 

Distance, Area, and Size. — The moon's average distance from the 
earth is about 240,000 miles. The actual distance during a single 
month varies about 30,000 miles, causing a corresponding variation 
in its apparent size. 

The diameter of the moon is 2,163 miles, being about 27 per 
cent of the diameter of the earth. 

The surfaces of the moon and earth are to each other as the 
squares of their diameters, or as one to fourteen. Their volumes 
are to each other as the cubes of their diameters, or as one to 
fifty. 

Real and Apparent Motion of the Moon. — The apparent motion 
of the moon and stars by night and of the sun by day, is due to 
the earth's rotation from west to east. There is a real eastward 
motion of the moon, as may be seen by noting the position of 
the moon among the stars from night to night. 

Since the moon makes one complete revolution about the earth 
in about 27^ days, the eastward motion is about 13 degrees a day; 
and as the sun also appears to move eastward among the stars 
about 1 degree a day, the eastward daily gain of the moon is about 
12 degrees. This causes the moon to rise about 50 minutes later 
each day. 

Moon has no Atmosphere or Water. — The moon has no appreciable 
atmosphere. Its absence is shown by the fact that when the moon 
hides a star, the star disappears suddenly and not gradually, as it would 
if its light passed through an atmosphere. There seem to be no effects 
of erosion on the moon, which also goes to show that there is no at- 
mosphere there. If the moon ever had an atmosphere at any stage of 
its development it has lost it. If water existed on the moon it would 
evaporate during the long day there and form an atmosphere. 



THE MOON 



31 




Fig. 15. — Lunar Topography (Stellar Evolution) 



32 PHYSIOGRAPHY 

Moonlight Surface Markings. — Moonlight is but reflected sunlight. 
The surface markings on the moon are known to be due to a very uneven 
surface. The visible surface of the moon has an area about equal to 
that of South America, and nearly one-half of the area is covered with 
dark gray patches which were once supposed to be seas. The rest of 
the surface consists of mountains, so called volcanoes and craters, and 
ringed valleys. Some mountain chains have peaks nearly 4 miles high. 

Same Face is Always Toward the Earth.— Since the same side of the 
moon is always turned toward the earth, it follows that the period of 
rotation of the moon on its axis and its period of revolution about the 
earth are the same, about 275^ days. Consequently we know nothing 
except by inference about the other side of the moon. The side of the 
moon that is toward the sun is always brightly illuminated, and the side 
turned away from the sun is in darkness. As the moon makes her way 
eastward around the earth, varying portions of the illuminated half are 
seen. This causes the moon's phases. 

PHASES OF THE MOON 

New Moon. — When the moon and the sun are on the same side of the 
earth, the dark side of the moon is turned toward the earth and we have 
new moon. New moon, strictly speaking, occurs when none of the bright 
surface is visible. Popularly the moon is said to be new when seen as a 
very thin crescent. A day or two later, when the moon has moved a 
little eastward of the sun, we may see in the early evening in the 
western sky a small portion of the illuminated half in the form of a 
crescent, convex westward, or toward the sun, with the horns turned 
eastward, or away from the sun. 

First Quarter. — A week after new moon, half of the illuminated hemi- 
sphere may be seen. The moon has now reached first quarter, and its 
shape is that of a half-circle. A line connecting it with the earth is at 
right angles to a line connecting the sun and the earth. As the moon 
passes beyond the first quarter the boundary line between the light and 
the dark area begins to be convex eastward, and the lighted portion con- 
tinues to grow larger. 

Full Moon. — When the moon and the sun are on opposite sides of the 
earth, the whole lighted half of the moon is turned toward the earth, 
and we have full moon, about a week after the first quarter. The line 
dividing the light and dark areas after full moon changes from the left 
side to the right side of the moon's disk. 

Third Quarter. — The moon reaches the last or third quarter about a 
week after full moon. In this phase the half-circle is convex toward 



THE MOON 33 

the left instead of convex toward the right, as seen in the first quarter. 
After third quarter, the moon being west of the sun, the crescent curves 
to the left or toward the sun, and horns point to the right away from 
the sun. 

Waxing and Waning. — In its revolution from new to full moon, the 
visible illuminated area increases and the moon is said to wax. From 
full to new the illuminated area decreases and the moon is said to wane. 




NEW CRESCENT 



LAST QUARTER 

Fig. 16. — Moon's Phases 
The real illumination of the moon is shown in the inner eight positions in orbit about the earth 
at E. The sun is at the right. The apparent illumination is shown in the corresponding 
outer position. 

Earth Shine. — The dark portion of the moon is sometimes lighted by 
sunlight reflected from the earth, called Earth Shine. This occurs at 
the young and old crescent phases, and makes the entire disk of the 
moon visible. 

ECLIPSES 

Shadows. — All of the planets and their satellites are opaque 
bodies and cast long, cone-shaped shadows away from the sun. 
The length depends upon the size of the sphere and its distance 
from the sun. The average length of the earth's shadow is 
about 866,000 miles, and that of the moon 232,000 miles. 



34 PHYSIOGRAPHY 

Cause of Eclipses. — The word eclipse as here used means a 
darkening of a heavenly body. This darkening may be real or 
apparent. The moon is eclipsed when it passes into the earth's 
shadow ; the sun is eclipsed when the moon passes between it and 
the earth. During a lunar eclipse the moon is really darkened, 
light from the sun being cut off by the earth. During a solar 
eclipse the sun is only apparently darkened; the moon cuts off 




a -Total Solar 
Eclipse 
b-Pnnial Solar 
Eclipse 

Fig. 17. — Solar and Lunar Eclipses 

light that would otherwise reach the earth. In reality it is the 
earth rather than the sun that is eclipsed. 

Total Lunar. — In the figure, the moon is passing through the 
earth's shadow, BCD, and is totally eclipsed. The moon's disk at 
this time is usually visible, however, because of sunlight bent into the 
earth's shadow by our atmosphere. This gives the moon during a total 
eclipse, a dull, copper colored appearance. 

Partial Lunar. — When the moon passes slightly north or south of the 
center of the earth's shadow, and only a part of the moon's disk enters 
the shadow, a partial lunar eclipse occurs. The moon in its monthly 
revolution about the earth usually escapes the earth's shadow entirely. 

Total Solar. — When the moon passes between the earth and the sun, 
and its shadow, called the umbra, reaches the earth, a total eclipse occurs 
in that portion of the earth covered by the shadow. 

Partial Solar. — Just outside the umbra of the moon's shadow, an 
observer in the penumbra or partial shadow would see only a part of 
the sun's disk, and would experience a partial solar eclipse. 

When the moon's shadow is not long enough to reach to the earth, 



THE MOON 35 

and the moon passes centrally across the sun's disk, leaving a ring of the 
sun's disk exposed, the eclipse is said to be annular. The moon appears 
as a black spot covering the central portion of the sun's disk, surrounded 
by a ring of light. 

Number of Solar and Lunar Eclipses in a Year. — There are always 
at least two eclipses of the sun in a year, and there may be as many as 
four. The largest number of lunar eclipses in a year is three. As every 
eclipse of the moon is visible at one time from all points on one-half of 
the earth, and eclipses of the sun from a narrow area only, many more 
lunar than solar eclipses are visible at a given place. 

QUESTIONS 

t. Compare the moon with the earth in respect to size and physical 
conditions. Where and when do we see the young crescent? The old 
crescent? How long is each usually visible? Why? 

2. During what phase of the moon do lunar eclipses occur? Solar 
eclipses? 

3. How many solar eclipses would occur each year if the orbits of the 
earth and moon were in the same plane? 

4. The time from full moon to full moon, called a lunar month, is 
2 9;Mj days, while the actual time of a revolution of the moon about 
the earth is 27^ days. To what is this difference due? 



CHAPTER IV 
THE SOLAR SYSTEM 

Solar System Defined. — The sun, together with the bodies 
revolving about it, is called the Solar System. The members of 
the system are the sun, the planets and their satellites, the plan- 
etoids, some comets, and meteors. They may be briefly described 
as follows: 

i. The sun is near the center of the S3^stem, a very large, hot, 
self-luminous body giving heat and light to the other members. 
Its gravitative attraction controls their motions. 

2. The planets, eight in number, upon one of which we live, 
revolve about the sun in elliptical orbits, in different periods of 
time, and at different distances from the sun. Planets are distin- 
guished from stars by their changing position among the stars, and 
by their visible disk when seen through a telescope. Stars keep 
their relative position in the sky, and through a telescope appear 
as points of light. 

Consult the following table: 



Planets 


Diameter 

in 

Miles. 


Average Distance 

from Sun in 
Millions of Miles 


Period of 

Revolution 

in Years. 


Number of 
Satellites 
or Moons 


Mercury 

Venus 

Earth 

, _ n v \ Mars 


2,700 
7,800 

7,9i3 
4,3oo 
87,000 
72,000 
35, 000 
32,000 


36 

67 . 
93 
141 

483 

886 
1,782 
2,792 


0. 24 

0.62 

1 .00 

1.88 

12.00 

29.00 

84.00 

165.00 




1 
2 


"Jupiter 

Saturn 

Uranus 

Neptune 


8 
10 

4 

1 



3. All except two of the planets have satellites revolving about 
them. The satellites are very unevenly distributed among six of 



THE SOLAR SYSTEM 37 

the planets, as seen in the table above. Our moon is an example of 
a satellite. 

4. The planetoids (planet-like bodies), more than five hundred 
in number, are small bodies, as compared with any of the planets, 
and revolve about the sun between the orbits of the planets Mars 
and Jupiter. 

5. Comets are bodies that are temporarily visible, of large 
dimensions and small mass, unstable in form, usually with long 
tails and with uncertain orbits. Some comets revolve about the 
sun in closed orbits, have fairly definite periods of revolution, and 
are consequently members of the Solar System. Other comets 
with open orbits enter and then pass out of the Solar System 
without becoming members of it. 

6. Meteors are comparatively small masses of stone or metal 
that enter the earth's atmosphere from outside space. The light 

^ or , J^pltrr Saturn Uranus Neptune 

Etinh ~"N. \ 

„V<7I1M \ \ 

Hemunr i 



\1 

Fig. 18. — Diagram of Orbits of the Planets Drawn to Scale. 



given out by them is due to their being heated by the friction and 
compression of the air. Meteors are popularly called " shooting 
stars." 

Size of the Solar System. — It will give us a better conception 
of the size of the orbits of the different planets if we draw to scale 
a map of the solar system. The orbits of the first four planets are 
so small compared with the orbits of the last four, that it is 
difficult to find a suitable scale to use, to represent the whole 
upon a single page of this book. The scale, one millimeter, 
equivalent to 20,000,000 miles, is used. 

Although the orbits of the planets are elliptical, they differ so 
little from circles that for this purpose the circle may be said to 
represent the planet's orbit. 

Space Outside the Solar System. — The known bodies occupying 
space outside of the orbit of Neptune are comets, meteoric swarms, 
large gaseous masses called nebulae, and stars. 



38 PHYSIOGRAPHY 

In literature the stars are often referred to as " numberless " 
and " countless." As a matter of fact, only about 3,000 stars 
can be seen without a telescope at any one time, and in the whole 
heavens there are fewer than 6,000 stars that may be seen with 
the naked eye. With the telescope fainter stars are seen. The 
moderate sized photographic telescope, with the modern sensitive 
plate, will show stars that are too faint to be seen with the largest 
telescopes. It has been estimated that the photographic plate 
has made record of about one hundred million stars. Each of 
these stars shines by its own light and is consequently a sun. 
Many are more brilliant and larger than our own sun, and may 
be centers of other systems. 

THE SUN 

Diameter, Density, and Temperature of the Sun. — The sun is a 
huge sphere of incandescent gases and metallic vapors, with a 
diameter of 866,000 miles, and is 1.4 times as heavy as a sphere 
of water of the same size. Although but a small fraction of the 
total light and heat given out by the sun reaches the earth, yet 
nearly all life activities and most movements of air and water 
are due to this amount. 

The difference between conditions on the sun and those now on 
the earth is due largely to a difference in temperature 

THE CONSTITUTION OF THE SUN 

The Photosphere. — The visible surface of the sun is called the photo- 
sphere (light-sphere). It is cloud-like in appearance and gives forth 
most of the light and heat which the sun radiates. 

Sun-Spots. — Dark spots of irregular outline, called sun-spots, often 
many thousands of miles in diameter, mar at times the brightness of 
the photosphere. The sun-spots are probably connected with the hidden 
circulation in the great body of the sun below the photosphere, and are 
dark only in comparison with it. Observers of sun-spots soon found 
that the sun turns on its axis from west to east. The earth's magnetism 
is disturbed during a period of unusual activity in the sun. A large 
number of sun-spots appear and a greater development of solar prom- 
inences occurs most frequently at these times. The period of maximum 
disturbance occurs on an average about every eleven years. 



3 £ 

■d 




THE SOLAR SYSTEM 39 

As the sun rotates on its axis in about 26 days, no spot would 
remain continuously visible for more than 13 days, being one-half of the 
period of the sun's rotation. Some spots last, however, only a few days, 
while others persist for months. 

Elements in the Sun. — By means of an instrument called the spectro- 
scope it is possible to tell some of the substances of which the sun is 




composed. About 40 elements, such as iron, carbon, hydrogen, nickel, 
silver, etc., familiar to us on the earth, are now recognized in a layer of 
gas overlying the photosphere. 

Chromosphere. — Outside this metallic layer is a deep envelope of 
gas, mostly hydrogen, called the chromosphere (color-sphere). 

When the moon comes between the earth and the sun, the light 
from the photosphere is cut off and the sun is said to be eclipsed. 
During the solar eclipse the chromosphere can be seen as a brilliant 
scarlet ring. From its surface tongues of flame called prominences shoot 
out to altitudes of many thousands of miles. 

The Corona. — The outermost portion of the sun . is the corona 
(crown), a halo of pearly light extending out many thousands of miles, 
with streamers reaching out millions of miles. It is believed that the 



40 PHYSIOGRAPHY 

light of the corona is due to the reflection of light from dust particles, 
liquid globules, and small masses of gas. 

How the Sun's Heat is Maintained. — The theory that the sun's heat 
is largely maintained by the gradual shrinkage of its volume is generally 
accepted. The fall of matter toward the center would continuously 
generate heat, as the blow of a hammer on a nail would heat both nail 
and hammer. Other sources of heat may add to the total amount that 
the sun sends out into space, such as that resulting from combustion, 
the falling of meteors, and radioactivity. 

THE PLANETS COMPARED 

The characteristics common to all of the planets may be briefly 
enumerated as follows: 

i. The planets move in the same direction about the sun from 
west to east. The sun rotates in this direction. The direction of 
movement as seen from above the north pole of the earth is oppo- 
site to the hands of a clock. 

2. The paths or orbits of all the planets are ellipses, with the 
sun at one of the foci. 

3. The other planets are non-luminous, like the earth; conse- 
quently the light that comes from them to us is reflected sun- 
light. 

4. Most of the planets are known to rotate in the same direction 
as the earth rotates, from west to east. 

THE PLANETS AS INDIVIDUAL BODIES 

Mercury. — So far as is known at present, Mercury is the smallest 
planet, the nearest to the sun, and the swiftest in its movements about 
the sun. It can be seen only in the direction of the sun during early 
twilight or late dawn. Mercury has a thin atmosphere, if any at all, 
has surface markings of permanent streaks, and a known rotation 
period equal in length to its year of 88 days. Since the periods of rota- 
tion and revolution are the same length, Mercury always turns the same 
side to the sun. This side is always heated and has perpetual daylight, 
while the side turned away from the sun is always cold and in darkness. 

Venus. — Venus shines in the sky with peculiar brightness. It has 
a diameter considerably more than double that of Mercury and only a 
little less than that of the earth. The period of rotation is now known 
to be 255 days, and equal to its period of revolution. Venus and Mer- 



THE SOLAR SYSTEM 41 

cury are the only planets that have equal periods of rotation and revo- 
lution. They pass between the earth and the sun, and consequently are 
the only planets that present all phases similar to those of our moon. 
The passages are called transits, and occur at irregular and relatively 
long intervals of time. During these passages Venus and Mercury 
look like small, round, black spots passing across the sun. 

The Earth. — Although we know that the earth is a planet moving 
about the sun like the other planets, the earth seems to us to be a 
center about which the other heavenly bodies move. The earth has the 
general form of the other planets, that of a spheroid. It is the third in 
distance from the sun, and the largest of the four smaller planets 
whose orbits lie within those of the planetoids. The earth makes 366 
rotations during one revolution. 

Mars. — Mars, though having only a little more than one-half the 
diameter of the earth, resembles it in more respects than any of the other 
planets. Its period of rotation is 24 hours, 37 minutes, or a little more 
than our day. The inclination of its axis is about 24 degrees. There- 
fore, except for its greater distance from the sun, the days and change 
of seasons resemble those of the earth. 

Surface markings on Mars indicate to some astronomers snow fields 
and canals. There seems to be little doubt about the white polar 
caps that appear and disappear according to the season. It is not cer- 
tain, however, that they are fields of snow. 

Although we have as yet no foundation from which to make any 
positive statement concerning the inhabitants of Mars, it may be 
claimed that if any planet other than the earth is inhabited, it is 
probably Mars. Mars appears in our sky shining with a steady, pale 
red light. 

Jupiter. — Jupiter is the largest of all the planets, and, with the excep- 
tion of Venus, often the brightest in the sky. Surface markings on 
Jupiter are described as parallel belts and spots. Because of the lack of 
the permanency of the markings, they are thought to be due to a deep 
atmosphere surrounding the planet. From observations of the spots, 
it has been found that Jupiter has a rotation period of about ten 
hours, which is the shortest known of any of the planets. 

The circumference of Jupiter is about 1 1 times the circumference of 
the earth, and with a rotation period less than half of that of the 
earth; the rate of rotation at the equator of Jupiter is about 30,000 
miles an hour, nearly 30 times the rate of rotation at the equator of the 
earth. 

The outer four planets comprising the major group — Jupiter, 
Saturn, Uranus, and Neptune — are supposed to be of a higher temper- 



42 PHYSIOGRAPHY 

ature, of less density, and in not so advanced a stage of development as 
the four planets Mercury, Venus, Earth, and Mars, comprising the 
minor group. 

Saturn. — Saturn is distinguished from all the other planets by three 
thin, flat meteoric rings, easily visible through a small telescope, which 
surround it in the plane of its equator. The rings are together about 
40,000 miles wide, and the inner edge less than 6,000 miles from the 
planet. At distances ranging from a hundred thousand miles to nearly 
eight million miles from Saturn, are ten satellites, more than have yet 
been discovered belonging to any other planet in the Solar System. 

The surface markings on Saturn are not seen nearly so well as those 
on Jupiter, because Saturn is nearly twice as far from us. There are 
bright and dark belts, and at times faint spots. Saturn rotates on its 
axis in about ioj^ hours. Because Saturn has a density of about three- 
quarters that of water, it is believed to be largely in a vaporous con- 
dition. It may be seen shining in the sky with a steady yellowish 
light, with about the same degree of brightness as the brightest star. 

Uranus and Neptune. — Uranus was discovered in 1831, and Neptune 
in 1846. All the other planets were known to the Ancients. Uranus is 
a very faint object in the sky, and Neptune is invisible to the naked eye. 
Neptune is the most remote of the planets now known, and has the 
longest period of revolution, one year there being 165 earth years. It 
may be inferred that the physical condition of Uranus and Neptune is 
probably much the same as that of Jupiter and Saturn. The rotation 
period of Uranus, as indicated by surface markings, is between ten and 
twelve hours. These planets, being so far from the sun, receive a 
small amount of heat per unit area compared with that received by 
the earth. 

The Satellites of the Solar System Compared. — Previous to 1610 
the only satellite known was our moon. In that year Galileo first 
pointed his telescope to the sky and saw four large moons of Jupiter. 
Our moon is more than 2,100 miles in diameter, but not as large as any 
of three of the eight moons of Jupiter, and one of the ten moons of 
Saturn. The largest satellite of Jupiter is 3,558 miles in diameter, and 
considerably larger than the planet Mercury. The smallest satellites 
known are the two belonging to the planet Mars, both of which are 
probably less than ten miles in diameter. One of the two is only 5,800 
miles distant from Mars, and makes a revolution in less than eight 
hours, one-third of the time it takes Mars to rotate. 

The earth's satellite is about 240,000 miles distant from the earth, 
and makes a complete revolution in about 27^3 days. The most dis- 
tant satellite of Saturn takes considerably more than an earth year to 



THE SOLAR SYSTEM 43 

make a revolution. The mass of our moon as compared with the mass 
of the earth is probably greater than the mass of any other single 
satellite, compared with the mass of its planet. Mercury and Venus 
have no satellites, Uranus has four, and Neptune one. 

The Planetoids. — The planetoids, sometimes called asteroids, move 
about the sun just as the planets do. They are so small that they 
are invisible to the naked eye. Not until the beginning of the nine- 




Fig. 21. — Halley's Comet (Evening Sky Map) 

teenth century were any of these bodies discovered. There was an 
early belief that an undiscovered planet revolved between the orbits 
of Mars and Jupiter. This, no doubt, led to the discovery of the 
first and largest planetoid, Ceres. 485 miles in diameter. There are 
several whose diameters are more than a hundred miles, but the majority 
are much smaller, ranging down to about ten miles in diameter. New 
ones are now being found every year by the method of photography. 

Comets. — Comets are in strong contrast with planets in appearance 
and physical condition. Most of them enter the Solar System with 
orbits in the form of open curves, make one turn about the sun, and 
pass away, probably forever. 



44 



PHYSIOGRAPHY 



Of the few comets that belong permanently to the Soiar System, all 
have definite periods of revolution about the sun, varying from 3.3 years 
(Encke's Comet) to about 76 years (Halley's Comet). Halley's Comet 
last appeared during May, 1910. 

The typical comet is largely self-luminous, and is composed of a 
head and a tail. In the center of the head is a bright, star-like 
nucleus surrounded by faintly luminous matter, called the coma. The 




Fig. 22. — Peary Meteorite 
In American Museum of Natural History, New York 



tail acts like your shadow when you walk around a lamp. It always 
points away from the light. Some astronomers maintain that it is the 
pressure of sunlight that drives the gaseous molecules from the nucleus 
and thus forms the comet's tail. 

The head may have a diameter greater than that of the sun, with 
a nucleus as large as the earth, and the tail equal in length to the dis- 
tance of the earth from the sun. The amount of matter in a comet is 
very small, in most cases less than one millionth of that of the earth. 

The orbits of the planets are slightly elliptical and all are approx- 
imately in one plane; those of the comets are greatly elongated and 



THE SOLAR SYSTEM 45 

lie in every possible position. With the unaided eye it is a rare sight 
to see a comet. 

Halley's Comet has been pursuing its fixed orbit about the sun 
since the dawn of history, and undoubtedly long before. The accounts 
of many of its earlier appearances seem to indicate that it has been a 
conspicuous object. The last appearance, during May, 1910, was dis- 
appointing. This tends to show that the great comet has for ages been 
slowly disintegrating. 

Under the most favorable conditions the nucleus of Halley's Comet 
was brighter than stars of the first magnitude, the coma was a faint 
light, and the tail was a band of light about 8 degrees wide at its 
widest place and 1 20 degrees long. Stars were plainly visible through 
the comet's tail. It is believed that the earth passed through the tail 
on May 18, 1910. At that time there were no unusual manifestations 
seen, such as the falling of an unusual number of meteors, a glow of the 
sky, or the appearance of deadly gases, all of which had been predicted. 

Meteors.— The earth in its path about the sun encounters daily 
many millions of small bodies which enter its atmosphere from out- 
side space. On a clear, moonless night, one may see several an hour. 
They often appear at altitudes of a hundred miles, move many miles a 
second, give out light and heat, and are usually consumed before they 
reach the surface of the Earth. These bodies are called Meteors. 

The appearance of an unusual number of meteors, usually in August 
and November, is known as a Meteoric Shower. Sometimes bodies 
weighing from a few pounds up to several tons fall to the earth's surface 
unconsumed. Such bodies are known as Meteorites. Some are com- 
posed of nearly pure iron, with a little nickel. Most meteorites are 
composed of stone, often with traces of iron in them. About thirty 
of the different elements found in the earth have been found in 
meteorites. 

THEORIES CONCERNING THE ORIGIN AND 
DEVELOPMENT OF THE EARTH 

The Nebular Hypothesis of Laplace. — Many hypotheses have 
been proposed, but the one that has exercised the greatest influ- 
ence upon thinking people is the Nebular Hypothesis, as formu- 
lated by Laplace. This hypothesis maintains: 

1. That the matter of the Solar System was once a highly heated 
mass of gas called a nebula. 

2. That the form was a vast spheroid extending beyond the 
orbit of the farthest planet. 



46 PHYSIOGRAPHY 

3. That the nebula was in process of cooling, and the cooling 
caused shrinkage. An effect of shrinkage was to increase the rate 
of rotation, and this increased the equatorial bulge. 

4. That when the rotation increased to a certain speed, the 
centrifugal force at the equator of the spheroid equaled the 
attraction of the gravitation. Upon further cooling and contrac- 
tion, the equatorial portion separated from the great rotating 
mass, forming a ring resembling the rings of Saturn. 

5. That as the cooling and contraction of the spheroid con- 
tinued, additional rings were separated. The first ring gave rise 
to the outermost planet, and the later ones to the other planets 
in turn. 

6. That the central body was the sun. 

7. That each ring parted at its weakest point, and the matter 
was collected into a planet, which was hot and gaseous. 

8. That the cooling of the planet caused a contraction, which in 
turn increased the rate of rotation, and consequently the amount 
of bulging. Some of the planets followed the example of the 
parent nebula, and formed rings which became satellites. 

9. That as the cooling and shrinkage went on, the gases changed 
to a liquid and then to a solid state. In the case of the earth, the 
volume changed from a rotating mass extending to the orbit of 
the moon to its present size. 

10. That the more volatile material of the earth remained in a 
gaseous state, and formed our atmosphere, originally much deeper 
and of a higher temperature than now. As the atmosphere cooled, 
the water vapor condensed and formed clouds. As cooling con- 
tinued, rain fell and the ocean formed. 

THE PLANETESIMAL HYPOTHESIS 

During the last few years the planetesimal hypothesis has been 
formulated, and may be stated as follows: 

1. The hypothesis starts with a cold nebula, spiral in form, 
which is the most common type now seen. 

2. The spiral nebula consists of a central portion or nucleus, 



THE SOLAR SYSTEM 47 

which became our sun, with two arms starting from opposite 
sides and curved spirally about the nucleus or center. 

3. A significant feature of the spiral nebulae is the presence of 
numerous nebulous knots in the arms. These knots are the 




Fig. 23. — Spiral Nebula 
From Stellar Evolution 



denser portions of the nebula, and the nuclei of future planets 
and satellites. 

4. The knots or nuclei are surrounded by a nebulous haze, 
which is composed not only of gaseous particles but also of in- 
numerable solid or liquid particles. These small' bodies revolve 
about the center of the nebula like little planets, and are called 



48 PHYSIOGRAPHY 

planetesimals. The nuclei grow and become planets and satellites 
by the in-fall of planetesimals. The earth and moon were two 
companion nuclei of unequal size. 

5. The earth developed from a knot in an arm of a spiral nebula 
by the capture of outside planetesimals. The increasing gravita- 
tional compression of the interior produced the internal heat of 
the earth. 

6. Gases were held in the solid planetesimals as they are held in 
meteorites that now fall to the earth. As the growing earth be- 
came heated by internal compression, the gases were given forth 
gradually, thus forming an atmosphere about the earth. Until 
the earth had attained a mass greater than that of the moon 
(•^Y of the earth), its gravity was probably insufficient to enable 
it to hold the gases of an atmosphere such as we now know. The 
gases now issuing from volcanoes were occluded in the original 
planetesimals which formed the earth. 

7. When the earth had. reached such size that water vapor was 
held in the atmosphere in sufficient quantity to reach the satura- 
tion point, the water vapor began to condense, and then the ocean 
began to form. 

THE TWO HYPOTHESES CONTRASTED 

Nebular Hypothesis Planetesimal Hypothesis 

1. Nebula, hot and large, formed 1. Nebula, cold, formed two 
rings around central mass or arms around central mass or 
sun. sun. 

2. Rings became planets. 2. Nuclei or knots became plan- 

ets and satellites. 
2. Smaller rings separated from 3. Smaller knots were captured 
planets and became satellites. by larger knots and became 

satellites. 

4. Planets and satellites origin- 4. Planets and satellites origin- 
ally hot and large, gradually ally cold and small, gradually 
cooling and growing smaller. heating and growing larger. 

5. Outermost planet, Neptune, 5. Planets and satellites forming 
formed first and others at at same time. 

later periods. 

6. Earth always had an atmos- 6. Earth when small without an 
phere. atmosphere. 



THE SOLAR SYSTEM 49 

QUESTIONS 

1. Name points by which each class of bodies comprising the solar 
system differs from all the other classes. 

2. Compare the diameter of the sun with the diameter of the orbit of 
the moon about the earth. Compare the periods of rotation of the sun 
and moon. 

3. As far as is known, which planet has the shortest period of rota- 
tion? How do you account for this? 

4. Briefly compare physical conditions on each planet with those on 
the earth. What planets have you seen? What are the difficulties in 
finding favorable opportunities for seeing the planets? 

5. What purposes do the satellites seem to serve? What are some of 
the superstitions connected with our moon? What was the first dis- 
covery made by the telescope? 

6. Why are the stars generally invisible by day? How can we dis- 
tinguish stars from planets? 

7. Why are planetoids never seen with the naked eye? What dis- 
tinguishes meteorites? 

8. What are some of the peculiarities of comets? Describe a comet 
you have seen. 

9. According to the Nebular Hypothesis, what planet was formed 
first? Why are the outer planets larger than the inner planets? 

10. According to the Planetesimal Hypothesis, why are some planets 
so much larger than others? Which theory would require the longer 
time for the development of the earth? Why? 

n. What are two real motions of the sun? Describe two apparent 
motions of the sun and point out cause of each. 

12. Which of the heavenly bodies are self-luminous? 

13. Are any of the planets repeating a portion of the earth's history? 
What ones? Have any of the planets reached a more advanced stage in 
their development than the earth? Which ones? Explain. 



CHAPTER V 
MAP PROJECTION 

Map making is one of the most important arts, and every great 
nation has a body of men engaged in surveying and map making. 
In the United States the General Land Office has mapped most of 
the country in order to allot and sell the public domain. The 
United States Geological Survey is making an accurate large scale 
map to show geological and relief features, our navigable rivers, 
our lakes, and our coasts. On maps, then, we depend for the sale 
of our public lands, and the navigation of our rivers, lakes, and 
seas. 

A map is the representation of a portion of the surface of the 
earth on a plane. The portion represented is indicated by its lati- 
tude and longitude. The scale of a map is the ratio between the 
length of a line on the map and the actual distance the line repre- 
sents. The scale one mile to the inch is also a scale of ^-3-^17 
because 1 mile equals 63,360 inches. On the U. S. Topographic 
Maps the scale most frequently used is i^so - ^ about 1 mile to 
the inch. The mapping of large areas with their curved surfaces 
and poleward converging meridians presents difficulties that are 
met by certain devices called projections. 

Projection, in map making, is a method of representing the curved 
surface of the earth on a plane. One method is illustrated by pro- 
jecting (throwing) upon a screen the shadow of the frame of a 
half globe, with wires for meridians and parallels. The point from 
which the rays of light proceed, where the eye may be placed to 
view the globe to get the same effect, is called the point of projec- 
tion; the screen is the plane of projection, and the rays of light 
the lines of projection. 

Orthographic Projection. — When the point of projection is dis- 
tant and the lines of projection parallel and at right angles to the 



MAP PROJFXTION 



51 



plane of projection, the orthographic projection is formed. If the 
plane of projection is parallel to the axis of the globe the ortho- 
graphic equatorial projection is formed with the equator as a diam- 
eter, the parallels as straight lines nearer together toward the 
poles, the central meridian straight, but the other meridians curv- 

Method Equatorial Polar 



Orthographic : 




Fig. 24. — Orthographic Projection 

ing and nearer together toward the margin. If the plane of pro- 
jection is at right angles to the axis, bringing a pole to the center, 
the orthographic polar projection is formed. In it the parallels 
are concentric circles nearer together toward the equator, which 
becomes the outer circle. The meridians are straight lines radiat- 
ing from the center. This projection, accurate at the center only, 
becomes increasingly inaccurate toward the margins, where dis- 
tances are much shortened. It shows the actual appearance of the 
globe. 

Stereographic Projection. — When the point of projection is at one 
end of a diameter of the globe and the plane of projection is at right 




Fig. 25. — Stereographic Projection 



angles to that diameter, the stereographic projection is formed. An 
equatorial diameter gives an equatorial stereographic and a polar diam- 
eter, a polar stereographic. This projection, if accurate at the margin, 



52 



PHYSIOGRAPHY 



becomes increasingly inaccurate toward the center. Areas are repre- 
sented more accurately than in the orthographic projection and less 
accurately than in the equidistant. 

Globular or Equidistant Projection. — When the point of projec- 
tion is taken about 1.7 radii from the center of the globe instead of at 
the surface of the globe as in the stereographic, the globular projection 

B 



Equidistant ^J, 




Fig. 26. — Globular or Equidistant Projection 

is formed. It is also called the equidistant because the meridians are 
equidistant along a given parallel and the parallels are equidistant along 
a given meridian. It has a polar as well as an equatorial form. It is 
more accurate than the stereographic projection and much more 
accurate than the orthographic. 



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Fig. 27. — Cylindrical Projection 



Cylindrical Projection. — The cylindrical projection is made upon 
a cylinder touching the globe at the equator only. The center of 
the globe is the point of projection. When the paper is slit along 
a meridian and unrolled the meridians and parallels appear as 
straight lines at right angles to each other, but at their true dis- 



MAP PROJECTION 



53 



tances apart at the equator only. The advantage of this projection 
is that nearly the whole earth is shown. The disadvantage is that 
there is excessive exaggeration of distances toward the poles and 
no uniform scale. This projection is often confused with the 
Mercator projection which has supplanted it. 

Mercator's Projection. — This is the cylindrical projection so 
modified that at every place the degree of latitude and the degree 
of longitude have the same ratio to each other as on the globe itself. 
This projection is used to show the whole surface of the earth. 
Mariners have adopted it because it shows directions correctly. 
Its disadvantages are that distances near the poles are greatly 
exaggerated and the scale is not uniform. (See Fig 32, page 60.) 




Fig. 28. — Mollweide Projection 



Mollweide Projection. — In this projection the equator and a merid- 
ian are laid off their true relative lengths at right angles to each other 
at their midpoints. The true relative distances between parallels are 
laid off along the meridian and the true relative distances between 
meridians along the equator. Ellipses are then drawn passing through 
the poles and the proper points on the equator to represent the meridians. 
The parallels are then drawn parallel to the equator. This projection 
is being used more and more. It is pleasing to the eye and has the great 
advantage of showing the entire earth. There is a slight exaggeration 
in polar regions, and quite a distortion of shape. 

Conical Projection. — In the conical projection the point of pro- 
jection is the center of the globe and the projection is made upon a cone 
touching the globe along any desired parallel. The cone is then slit 



54 



PHYSIOGRAPHY 



along a meridian and spread out. It is evident that this projection is 
accurate at the parallel of contact, becoming very inaccurate toward 
the poles, one of which cannot be shown. 

By using different parallels of contact as bases, it is possible to map 
large areas accurately on a large scale. This poly conic projection is 
used in the United States Topographic Maps. On such maps the top 
parallel is slightly shorter than the bottom one. 



Nortli Pole 




Fig. 29. — Conical Projection, Based on Parallel 30° North 



GLOBES AND MODELS 

The surface of the earth can, in some ways, be best represented 
by maps, in other ways best by models or by globes. Globes have 
the advantage of representing the whole earth, in its exact shape, 
and with all regions in their true relative positions and in their 
true relative sizes. Globes are generally on too small a scale to 
show much detail. 

Elevations and depressions of the surface of the earth are tech- 
nically known as relief. Relief is best represented by models. The 
relief of the earth is relatively so slight that models of large areas 
fail to give a correct idea of the surface unless an exaggerated 
vertical scale is used. This is because horizontal distances on the 
landscape are foreshortened, whereas vertical distances are not. 



MAP PROJECTION 
TOPOGRAPHIC MAPS 



55 



Maps on which the physical features are represented are called 
topographic maps. On the United States Topographic Maps water 
features are represented in blue; culture features, the work of man, 
in black, and relief features in brown by means of contours. 




Fig. 30. — Contour Map of an Island with Three Profiles 

The number of spaces between contours shows that point C is 5 contour 
intervals above sea level, and / is 2 1 9. 

From C toward N E the contours are close together. The profile indi- 
cates that the slope is steep. 

From C toward 5' E the contours are far apart, indicating a gentle slope. 

From C toward S W the contours are equidistant, indicating a uniform 
slope. 

From C toward W the slope is gentle, becoming steep, and is convex 
to sky. 

From C toward E the slope is steep, becoming gentle, and is concave 
to sky. 

The re-entrant contours along line C V indicate a valley. 

The outcurving contours along C W indicate a ridge or spur. 

Contours are lines connecting places of equal elevation. Each 
one shows where the new shore line would be if the sea level should 



56 PHYSIOGRAPHY 

change a certain distance vertically. This vertical distance be- 
tween adjacent contours is called the contour interval. On most of 
our Government maps the contour interval is 20 feet; but it is only 
5 feet on certain portions of the flood plain of the Mississippi 
River, and is sometimes 250 feet in mountains that are very high 
and steep. The significance of contours is brought out by cross 
sections called profiles. 

Hachures. — Relief is also represented on maps by differences in color 
and by means of hachures. Hachures are lines drawn to represent the 
path water would follow in flowing down a slope. There are many differ- 
ent systems of hachures; in one much used the lines are short and thick 
where the slope is steep and long; fine and far apart where the slope is 
gentle. (See Fig. 143, page 292.) 

QUESTIONS 

1. Compare the advantages and disadvantages of maps, models, 
and globes. 

2. Why are map projections necessary? 

3. Compart the advantages and disadvantages of three projections. 

4. Name the projections used in the various maps of this book. 

5. Note carefully the method of projecting and draw, with a 6-inch 
diameter, an orthographic equatorial projection of the globe you use, 
numbering the meridians and parallels. Trace in one of the continents, 
as South America or Africa, from the globe. 

6. Proceed similarly for orthographic polar projection. 

7. Proceed similarly for cylindrical projection. 

8. Choose those pupils who have done the best work to construct on 
large sheets of manila paper large scale maps to be hung on school- 
room walls when needed. The backs of maps already mounted may be 
used for this. Waxed crayons or colored chalk crayons dipped in melted 
wax are cleaner than ordinary colored chalk. 

9. If in Fig. 30 the contour interval is 20 feet, how high is the point C? 
How long is the island if the vertical and horizontal scales are the same? 
Draw a profile through the center from N N W to S S E, and another 
from W N W to E S E. 

10. Put into a basin a stone shaped like a mountain and fill the basin 
so that the tip of the stone just shows. Draw the location of this point 
very carefully on a piece of paper placed beside the basin. Lower the 
level of the water an inch and draw very carefully the shoreline of the 
stone. Remove another inch and so continue. Draw to the same scale 



MAP PROJECTION 57 

as in drawing a view of the stone from one side. Label the view and 
the contours. 

11. Trace your contours very lightly on another piece of paper, using 
carbon paper or holding the papers against a window. Change the con- 
tour map to an hachure map. 

12. Using the same color scheme as on a United States Topographic 
Map, show by contours, etc., two peaks of different height, a river, a 
lake, a steep slope, a gentle slope. Label properly and locate two points 
A and B in sight of each other, and two other points X and Y not in 
sight of each other. 



CHAPTER VI 
TERRESTRIAL MAGNETISM 

Space about magnets is known as the Magnetic Field. If a 
small magnet, known as a magnetic needle, is carried into the 
magnetic field of a large steel magnet or an electro-magnet, the 
needle will turn and set itself in a definite position in relation to 
the magnet. It has been found that the whole earth is surrounded 
by a magnetic field, and that magnetic needles set themselves 
in definite directions in relation to the earth. If we should follow 
the direction in which the magnetic or compass needle points, 
we would be going along a magnetic meridian. These magnetic 
meridians converge and meet in a locality north of Hudson Bay, 
latitude 70 N., and longitude 97 ° W., known as the XortJi Mag- 
netic Pole; and also in the Antarctic regions in latitude 72 ° S., 
and longitude 150 E., known as the South Magnetic Pole. 

The north magnetic pole of the earth being 20 degrees from the 
geographic north pole and the south magnetic pole about 18 
degrees from the geographic south pole, it is seen that the magnetic 
meridians do not have the same direction as the meridians of 
longitude. It follows that the north-seeking end of the compass 
does not indicate true north in most places on the earth. The 
departure or variation of the needle from a true north is called 
magnetic declination. 

Lines connecting places having the same declination are isogonic 
lines, and lines connecting places of no declination are agonic 
lines. There are many isogonic lines drawn on magnetic charts 
of the world, but only three agonic lines. One agonic line crosses 
the United States from Lake Superior, through Ohio and Kentucky 
to South Carolina. On this line the compass needle points due 
north. At all places in the United States east of this line, the 



TERRESTRIAL MAGNETISM 



59 



needle points west of north. West of this agonic, at all places in 
the United States, the compass needle points east of north. 

In the state of Maine the variation of the needle is more than 
2o° west: in the state of Washington more than 20 east, and 
in Alaska more than 30 east. 

By consulting map (Fig. 32) for the magnetic variation of any 
place and then making the necessary correction, the compass may 



' > K^^--{ £ Y. I > t - -* "'• \ 




Fie. 31. — Location - of the Xorth Magnetic Pole 



be used for determining true north. Explorers find the magnetic 
needle of little value in pointing out direction in unmapped re- 
gions, such as areas about the Xorth and South Poles. 

The Mariner's Compass. — This instrument consists usually of 
several magnetic needles placed side by side, fastened together, 



6o 



PHYSIOGRAPHY 




TERRESTRIAL MAGNETISM 6l 

and placed under a circular card. The needle and card are placed 
in a basin and supported at the center upon an agate point. The 
whole is suspended in such a way that it is always in a horizontal 
position, nothwithstanding the rolling of the ship. 

Inside the compass box is a black line called the Lubber Line, 
placed in the direction of the ship's bow. The compass card con- 
tains 32 rays, each indicating a direction or point of the compass. 
Naming the 32 points is called " boxing the compass." 




Fig. 33. — Mariner's Compass 



PART II 

THE AIR 



CHAPTER VII 
PROPERTIES AND FUNCTIONS OF THE AIR 

Introduction. — No part of his environment is of more immediate 
concern to man than the air he breathes. If it is pure he is strong. 
Vitiate it and he sickens. Withdraw it, but for a single hour, and 
he dies. 

No other part of his environment has had so great an influence 
in helping or retarding him in his struggle for existence or in his 
effort to improve his condition. How he dresses, what he produces, 
and what he eats are matters chiefly of weather and climate. Too 
great heat and too great cold are alike prohibitive of higher aspira- 
tions for better things. 

The savage Blacks of equatorial Africa and the Eskimo of 
the frozen North are both low in the scale of civilization; the first 
because the enervating climate destroys ambition; the second be- 
cause providing for mere physical needs exhausts his energies, 
leaving no opportunity for cultivation of the higher qualities. 
Both must adapt themselves to their climatic environment ; 
neither can change it. 

Definition. — The earth's atmosphere, or air, is the outer gaseous 
part of the earth. It envelops the solid and liquid parts, extend- 
ing to a height of probably more than two hundred miles, and fills 
all mines, caveSj and underground passages. As ground-air it 
penetrates all soils, and by the movements of the water it is car- 
ried to the greatest depths of rivers, lakes, and seas. 

Properties. — Pure air is an invisible gas, colorless, odorless, and 
tasteless; very compressible and perfectly elastic. It is very 
mobile, and like all matter, has weight. Though under ordinary 
conditions gaseous, it may easily be made to assume the liquid 
state. 



66 PHYSIOGRAPHY 

The compressibility and elasticity of the air make possible its 
substitution for steam in driving machines. This use is par- 
ticularly important in deep mines, where the long distances it must 
be carried results in condensation of the steam. 

The inertia of the air causes a resistance to motion through it, 
retarding the speed of the runner, the automobile, and the express 
train. When the air is in motion its inertia causes pressure on 
objects not moving with it, which varies as the square of the 
velocity. 

Composition. — Air is essentially a mechanical mixture of nitro- 
gen, oxygen, carbon dioxide, and argon. Water vapor, water 
particles and dust are usually present in it. The relative amounts 
of the first four are nearly constant, while the last three are ex- 
tremely variable. 

Nitrogen and oxygen bear to each other about the ratio of 78 
to 21 by volume, and 76 to 23 by weight. Carbon dioxide con- 
stitutes about three hundredths of one per cent of the air, varying 
slightly with locality and season. Of argon little is known, its 
existence not being known until within recent years. Argon con- 
stitutes about one per cent of the air. It was formerly included 
with nitrogen. 

Essential Composition of the Air 

Nitrogen 78 . 00% 

Oxygen 21 . 00% 

Argon 1 . 00% 

Carbon Dioxide 03% 

Distribution of Components. — In obedience to the principle of 
diffusion (ready and spontaneous mixing) the gases of the air 
make a fairly uniform mixture. Local conditions may tempo- 
rarily disturb this adjustment, but on the whole the air of one 
region of the earth is like that of any other. 

Carbon dioxide, being one of the products of volcanic action, is 
most abundant in regions of active volcanoes. Being likewise a 
product of decomposition and combustion, it is more abundant in 
cities, especially in manufacturing cities, than in the country; and 
more abundant in winter than in summer. The use by growing 



PROPERTIES AND FUNCTIONS OF THE AIR 67 

plants of carbon dioxide tends to decrease still further its summer 
percentage. 

Water vapor, though always present in the air, is not an essen- 
tial component. It is one of the most variable constituents of the 
air, and is in general more abundant over the sea than over the 
land, in low than in high altitudes, and in summer than in winter. 

Water and ice particles in the air, known as cloud, fog, mist, 
rain, snow, hail, and sleet, are limited to the lower air, reaching 
an altitude of only a few miles. 

Dust in the air is of two kinds, organic and inorganic. Organic 
dust includes microscopic animals and plants, pollen, fibers of 
wood and cloth, and the soot of smoke. Inorganic dust consists 
chiefly of powdered minerals and rocks derived from the land and 
caught up by the winds. Dust is more abundant over the land 
than over the sea, and is confined to the lower air. It is more 
abundant in cities than in the country, and in dry than in rainy 
weather, the dust particles being carried down by the falling rain 
drops. 

Mountain health resorts are sought partly because of the greater 
dryness of the air, and partly because of its freedom from dust and 
the disease germs that constitute part of the organic dust of the 
air at lower altitudes. 

Ozone, sometimes considered a constituent of the air, is really oxy- 
gen under peculiar conditions. By passing an electric spark through 
air the oxygen is in part changed to ozone, which, however, changes 
back to the more stable condition of oxygen. 

The invigorating quality of the air after a thunderstorm is thought 
to be due, in part at least, to the ozone produced by the passage of 
lightning flashes through it. The percentage of ozone increases with 
the altitude. 

Function of the Air. — Although the most important uses of the 
air are those of its individual components, yet the air as a whole 
has important functions. By virtue of it flight of birds and man 
is made possible, and sounds are transmitted. By air in motion 
ships and wind-mills are driven, life-giving and disease-producing 
germs are carried, and the seeds of many plants are fertilized and 
distributed. Rain is distributed over the lands, and waves and 



68 PHYSIOGRAPHY 

ocean currents are produced. Tornadoes and hurricanes, with all 
their destructive power, are but air in violent motion. 

As a carrier of waste from higher to lower levels, thereby wear- 
ing down the lands, and in the accumulation of sand dunes and 
loess deposits, the air is an important geological agent. Its 
presence in the mantle rock promotes disintegration of the min- 
erals and the production of soil. 

One of the chief purposes in cultivating crops is to increase the 
amount of ground-air. When the surface is packed by rains and 
remains unbroken by cultivation, air penetrates the soil with diffi- 
culty, and growing crops languish. 

Function of Oxygen. — The oxygen of the air is the supporter of 
combustion. By its chemical union with other elements heat is 
evolved. This process, called oxidation, may be slow, as in the 
rusting of metals, in which case the heat radiates as rapidly as 
produced, and there is no perceptible increase of temperature; or 
it may be rapid, as in the burning of wood, coal, or oil, resulting 
in an increased temperature, and often in the production of light. 
By combination with carbon in the blood of animals oxygen sup- 
plies the heat necessary to animal life. 

The readiness with which oxygen unites with most other chem- 
ical elements makes it active in promoting the disintegration of 
rocks and minerals. It is an important agent in the decomposi- 
tion of dead animal and vegetable matter, thus serving as a purifier 
of the air. In the form of ozone its activity is increased. 

Oxygen is more soluble in water than are the other constituents of 
the air. The percentage of oxygen in air enmeshed in water is therefore 
greater than in ordinary air, its ratio to nitrogen by volume being 34 
to 66 instead of the ordinary ratio of 21 to 78. It is this enmeshed air, 
obtained at the surface and carried by currents to the greatest depths 
of all lakes and seas, that makes life possible, even in the profoundest 
deeps. 

Function of Carbon Dioxide. — The carbon dioxide of the air, 
though of no direct use to animals, is essential to the life and growth 
of plants. Through the action of sunshine and the chlorophyl, or 
the green matter, of the plant, carbon dioxide, absorbed mainly 



PROPERTIES AND FUNCTIONS OF THE AIR 69 

through the leaves of the plant, is broken up, the carbon retained 
and the oxygen returned to the air. The carbon thus obtained 
unites with other substances brought in solution in the sap, 
thus manufacturing plant food and contributing to the plant's 
growth. Dissolved in water, carbon dioxide contributes to the 
growth of aquatic plants. It is the most effective of the gases 
of the air in decreasing the intensity of the sun's rays, and in 
checking radiation of heat from the earth. 

Since plants use carbon dioxide in the day time it is well to 
have growing plants in the living room, the air on their account 
containing a slightly increased per cent of oxygen. On the other 
hand they should be excluded from sleeping apartments at night, 
since they use some of the oxygen and none of the carbon dioxide. 

When plants decay, or are burned, the carbon stored up in their tissues 
is returned, usually, to the air in the form of carbon dioxide. Under 
certain conditions, however, as submergence in water, or burial out of 
contact with the oxygen of the air, the carbon of the decaying plant may 
contribute to a future store of mineral fuel in the form of coal, oil, or gas. 

Function of Nitrogen. — Since nitrogen constitutes more than 
three-fourths of the weight of the air, without it the air would be 
less than one-fourth its present density. Flight for most forms 
would then be impossible, and moving air as an agent for driving 
machinery and wearing down the land would be correspondingly 
weakened. 

Another important function of nitrogen is its use as a plant 
food. It is a necessary element of the food of all plants, and like 
most other elements is taken through the roots in solution. If the 
soil is lacking in this element no plant will thrive. 

Unlike oxygen and carbon dioxide, nitrogen is not taken by the 
plant directly from the air as nitrogen, but comes by way of the 
soil from some soluble compound of nitrogen. Some nitrogen is 
obtained in the form of nitric acid, carried down from the air by 
falling rain drops. This supply was formerly supplemented chiefly 
by the application of fertilizers, often in the form of expensive 
nitrates imported from distant regions. 

We have learned, however, that certain plants, of the family to 



70 PHYSIOGRAPHY 

which the clovers belong, are " nitrogen gatherers." These plants 
serve as hosts for minute organisms, which, attaching themselves 
to the roots of the plant, gather and store upon the roots in little 
nodules the nitrogen from the ground-air. 

Cowpeas, clovers, vetches, beans, and alfalfa are now extensively 
grown, alike^ for their value as forage crops and for the nitrogen 
they add to the soil. The entire growth above ground may be 
removed and yet the soil be left richer in nitrogen than before the 
crop was grown. 

Function of Water Vapor. — The water vapor of the air is the 

source of clouds, fogs, and of all forms of precipitation. Without 
it the earth would become parched, and life impossible. It is 
lighter than dry air, and its presence makes the air lighter. Like 
carbon dioxide it absorbs insolation (radiant energy from the sun) 
and heat radiated from the earth. Condensed as cloud it is more 
effective in protecting the lands from the direct rays of the sun 
and in checking radiation of heat from the earth. Precipitated as 
rain it supplies growing plants with necessary water; and as snow 
retards radiation and protects crops from the intense cold of 
winter. This function of snow is very important in the wheat- 
growing regions of the Northwest. 

Function of Dust. — Perhaps the most important function of dust 
is its diffusion (irregular scattering) of light. Without such diffusion 
objects would be visible only by reflection, as they now are at 
night; and the change from day to night and from night to day 
would be sudden, without twilight or dawn. 

At certain seasons a considerable part of the dust of the air is 
plant pollen. Many plants require pollen from other plants, and 
without the wind-borne pollen these would not be propagated. 

Putrefaction and fermentation are largely due to the organic 
dust of the air. Flesh and vegetables in high altitudes do not 
decay readily but simply dry out and shrivel up. This is due to 
the freedom of the air from dust germs. Some Indian tribes in 
these regions mummify their dead by simply exposing them in the 
air and sunshine. 



PROPERTIES AND FUNCTIONS OF THE AIR 71 

The germs of many diseases are distributed as air-dust; and 
flesh wounds heal more readily when the dust germs are washed 
away and excluded from the wound. 

Every dust particle in the air is a nucleus about which water 
vapor may condense ; consequently dust in the air promotes cloud 
formation and rainfall. Some have even taken the extreme view 
that without dust in the air no rainfall would be possible; but 
this has been disproved by experiment. 

Of esthetic interest is the fact that the sky owes its beautiful 
and varying colors, for the most part, to the dust in the air. Gor- 
geous sunrises and sunsets occur when the air is laden with inor- 
ganic dust, or with the smoke from- forest fires. 

Origin of the Air. — If the Nebular Hypothesis concerning the origin 
of the Solar System be accepted, the air may be considered a remnant of 
a formerly denser atmosphere. In this earlier atmosphere many of the 
elements which now make up the lands and seas existed as gases in an 
intensely hot condition. With loss of heat by radiation these elements 
changed to the liquid or solid state. Many of the elements of the 
primitive atmosphere were thus withdrawn, leaving the present rem- 
nant, the air as we know it. 

If we accept the Planetesimal Hypothesis of the origin of the Solar 
System, we believe that the air has been driven out from the interior of 
the earth by the increasing temperature and pressure. The gases thus 
driven out escaped to outer space while the earth was small and its 
gravitative attraction weak, and remained as part of the earth only after 
the earth's attraction became strong enough to hold its gaseous envelope. 

Future of the Air. — Whatever the origin of our earth or of its gaseous 
envelope, the earth is continuously losing heat. We may therefore look 
forward to the time when it will have the temperature of outer space, 
excepting only the surface that is turned toward the sun. 

Experiment proves that most gases can, with sufficiently low temper- 
atures, be liquefied and solidified. We also learn, from a study of the 
other members of our system besides the earth, that the smaller ones, 
such as the earth's moon, seem to have no atmosphere. These smaller 
members have cooled most, and if they ever had atmospheres their 
present low temperatures have probably resulted in making their 
atmospheres part of their solid masses. 

We may therefore infer that with further loss of heat by the earth 
the terrestrial seas must in time become solid; and eventually the air 



72 PHYSIOGRAPHY 

itself become in turn liquid and solid. Upon such an airless earth 
life, as we know it, could not exist; and the earth would then appear, to 
an observer upon another planet, the lifeless globe that our moon now 
appears to us. 

QUESTIONS 

sr i. Why is it not correct to say "the air surrounds the earth"? 

2. How can you show that the air has weight? Tha t it penetrates the 
soil? 
_3. In what particulars is country air usually purer than city air? 

— 4. In what sense does rain purify the air? 

5. Why will plants thrive better than animals in hot, marshy lowlands? 

6. Why are trips to the mountains and sea voyages recommended for 
convalescents? 

/► 7. How can you prove that there is dust in the air; and how can you 
decrease the dust in your bed-chamber without stopping ventilation? 

8. Why will milk that has been heated before bottling not sour as 
quickly as that which is bottled without heating? 

9. Why do dairymen cool their milk before shipping; and why is ice 
used to keep milk sweet? What is the principle of "cold storage"? 

10. Why will a candle lighted and lowered into a narrow deep bucket 
so quickly be extinguished? Why are lamp burners ventilated? 

— 11. Why do we ventilate our houses? What would be the result if 
we did not? Explain the horror of the "Black Hole" of Calcutta. 

-— 12. How do you know there is water vapor in the air? 

13. Why should a wound be thoroughly cleansed before binding up? 
What is the principle of disinfection and sterilization? 
-» 14. Why is it necessary to thoroughly dry our steel cutlery, and not 

so necessary with our silverware and china? 
_, 15. Why do fires in open fireplaces and in stoves connected with flues 
burn better than fires built in the open air? 

16. How can the nitrogen of the soil be increased most economically ? 



CHAPTER VIII 
TEMPERATURE OF THE AIR 

Sources of Heat. — So evident is it that the sun is the chief 
source of heat that the statement of the fact seems to need no 
demonstration. The temperature of our days increases with in- 
creasing length of the period of sunshine and with the nearer 
approach of the sun to our zenith, whereas our coldest season is 
that in which the nights are longer than the days, and the sun's 
noon position is low above the horizon. The hot belt of the earth is 
that which receives nearly vertical rays, while the frozen regions 
near the poles have only slanting rays. 

At first thought the sun appears to be the only source of heat; 
yet we know there are other sources. One of these minor sources, 
the interior heat of the earth, is of considerable importance, 
notably in deep mines, and in the production of volcanos and 
hot springs. 

The surface of the land varies in temperature from day to 
night and from summer to winter; but if we descend below the 
surface the variation is less and less. A depth is finally reached, 
varying with the latitude, at which the temperature does not 
change, and below this depth the temperature grows warmer the 
deeper we go. On this account we conclude that the interior of 
the earth is intensely hot. On the other hand, if we ascend in 
the air we find that the temperature grows colder, and at the height 
of only a few miles freezing temperatures, even in summer, are 
reached. Reasoning from this basis we conclude that outer space 
is intensely cold. 

From our knowledge of cooling bodies we know then that the 
earth must be a cooling body, sending its heat in eyery direction 
into outer space, and bringing about equal amounts to every part 
of the surface of the land. 



74 



PHYSIOGRAPHY 



Unimportant amounts of heat are received from the stars, and 
reflected from the other planets and the moon. 

Insolation.— The radiant energy that comes to us from the sun 
is called insolation. It does not come to us as heat, but manifests 
itself in many ways, e. g. as light and electricity. Only the 
insolation which is absorbed by any body is changed to heat and 
warms the body. 

As solar energy passes out from the sun-center in all directions, it is 
evident that only a very minute fraction of it will be intercepted by so 
small a body as the earth, at an average distance of about ninety-three 
millions of miles. Of the amount thus intercepted but a small portion 
is absorbed and transformed into heat; yet upon this minute part of the 
total solar energy all of our life-interests and activities depend. 

Disposal of Insolation. — When insolation is received, it is dis- 
posed of in three ways: by reflection, by transmission, and by 
absorption. As before stated, it is only the absorbed insolation that 
affects the temperature of the body. 

Each kind and condition of matter disposes of insolation in a 
distinct way. Some substances are good reflectors, some good 
transmitters, and some are good absorbers. Experiment has shown 
that in general, gases are the best transmitters, liquids the best 
reflectors, and solids the best absorbers. 

The absorptive power of a body may be materially modified by 
a change of color or of surface. Dark colors and irregular surfaces 
generally promote absorption, while light colors and smooth sur- 
faces promote reflection. By increasing the reflecting power of a 
body we decrease its absorbing power. 

The following table sets forth, comparatively, the treatment of 
insolation received by land, water, and air: 





Land 


Water 


Air 


Reflector 

Transmitter 

Absorber 


Fair 
Poor 
Good 


Good 
Fair 
Poor 


Very poor 
Very good 
Poor 



TEMPERATURE OF THE AIR 75 

Loss of heat by radiation is in direct ratio to the absorbing power 
of a body; a good absorber being a good radiator, and a poor ab- 
sorber a poor radiator. If the reflecting power of a body be in- 
creased its radiating power will be lessened. Radiation is con- 
tinuous. 

Heat in a body may be distributed by passing from particle to 
particle in contact; this process is called conduction. Solids are 
mainly heated in this way, but differ widely in their power of 
conduction. 

In liquids and gases, e. g. water and air, the most important 
method of distributing heat is by convection. By this process parti- 
cles in contact with a heated surface are warmed and expand, and 
after expansion are lighter, volume for volume, than the surround- 
ing particles. The heavier particles then sink, under the greater 
pull of gravity, and the lighter are crowded away from the heating 
surface, the heavier being heated in turn. This process, depending 
as it does upon gravity, requires that the heating surface be below 
the substance to be heated. 

The principle of convection is applied in the heating of our 
houses and in the construction of flues and chimneys. 

The land and water, being heated from above, are never warmed 
to very great depths; while the air, being chiefly heated at the 
bottom, by contact with the land and water, is warmed more 
rapidly and through a much greater thickness. 

How the Air is Heated. — The power of absorption of the air, 
though small, increases with increase in density, increase in car- 
bon dioxide and water vapor, and in the number of dust and 
liquid water particles present. Each dust and water particle, 
being a better absorber than air, becomes itself a center of warm- 
ing. Therefore, when insolation comes to earth it passes through 
the rare upper air with little loss by absorption. As it penetrates 
farther into the denser and dustier air more and more of it is 
absorbed, and the air is more and more heated. The air absorbs 
from one-half to three-fifths, depending upon its cloudiness, of ver- 
tical insolation passing through it. 

The air is heated most at the bottom, not only because of the 



76 PHYSIOGRAPHY 

increased absorbing power of the lower layers, but also because of 
their contact with the warmer land and water surfaces. 

Another very important aid in the heating of the lower air is its 
convectional mixing. The air in contact with the warmer land or 
water surfaces is warmed and expands. The cooler, heavier air 
above sinks and takes its place, to be in turn warmed and re- 
placed by cooler air from above. This mixing is for the most part 
confined to a stratum of air five or six miles in thickness. As long 
as the land and water are warmer than the air resting on them, 
convectional mixing will continue a factor in the warming of the 
lower air. 

The convectional ascent of heated air may be observed above a 
lighted gas jet, a hot stove, or a bonfire. Our rooms may be ven- 
tilated by admitting cool air at the bottom and permitting the 
escape of the heated air above. 

How the Air is Cooled. — When insolation ceases, as at night, 
conditions are reversed. Absorption, in excess of radiation during 
the day, is at night exceeded by radiation, and the air is cooled. 
Not that radiation does not continue during the day, for it is 
greatest when the temperature is highest, but the air does not 
begin to cool until radiation is more rapid than absorption. 

Since a good absorber is a good radiator, that part of the air 
which was most heated during the day is most cooled when inso- 
lation ceases. As a consequence, the rare, upper air is but little 
cooled, while the lower air is cooled most. Each dust and water 
particle, a center of warming during insolation, becomes a center 
of cooling when insolation ceases. 

One important factor in the warming of the air, convection, is 
wanting when the air begins to cool. Being most cooled at the 
bottom, the lower layers of air are heaviest, hence there is no 
tendency toward convectional mixing. In order to have cooling 
by convection it would be necessary to have the air cooled most 
at the top. On this account the lower air warms up faster than it 
cools down. 

The coldest hour of the day is from 4 to 6 a. m., and the warm- 
est from 1 to 3 p. m., depending upon the season. Thus it takes 



TEMPERATURE OF THE AIR 



77 



from seven to nine hours for the air to warm up, while from fifteen 
to seventeen hours are required for it to cool down. 

Temperatures Determined. — The temperature of the air, with 
reference to certain chosen temperatures, is determined by the 
thermometer. The temperatures of reference are those at which 
pure water freezes and boils under a pressure of approximately 
14.7 pounds to the square inch. This is the average pressure of 
the air at sea level. 

The action of the thermometer is based on the fact that most 
substances expand uniformly with heating, and contract uniformly 
with cooling. The measure of expansion or contraction may be 
taken as a measure of the amount of heating or cooling. 

Two general classes of thermometers are made, 
liquid and non-liquid. Almost any liquid or metal 
may be used. In the United States and other 
English-speaking countries, two scales for thermom- 
eters are in common use: the Fahrenheit (F), and the 
Centigrade (C). Their relation to each other and the 
method of converting readings of one to readings of 
the other are shown in the accompanying figure and 
table. 

Fig. 34 shows both Fahrenheit and Centigrade scales. 
It will be observed that the two scales agree at — 40. 
Freezing point is 32 on the F., and o on the C, and 
boiling point 212 and 100 respectively. It will thus 
be evident that a change from 32 to 212 degrees on 
the F. thermometer is equivalent to a change o to 
100 on the C. This relation may be thus expressed: 
180 F = ioo° C 
9° F - 5° C 
i°F= %° C 
t.8° F = i° C 



Fig. 34 



History of the Thermometer. — The thermometer was invented by 
Galileo, early in the seventeenth century. Soon after its invention it 
was graduated into 360 parts, corresponding to the number of degrees 
in a circle, hence the name degrees applied to these divisions. The name 






78 PHYSIOGRAPHY 

has been retained for the divisions of modern thermometers, though very 
differently and variously graduated. It was never significant. 

Fahrenheit was the first to adopt definite temperatures as a basis for 
graduation. According to his scale the boding point of water was found 
to be 212°, and the freezing point 32°. In the Centigrade thermometer 
100° is taken as the boiling point and 0° the freezing point. 

The accuracy with which the instrument may be read depends upon 
the length of the degree, and this in turn depends upon the relative 
capacities of bulb and tube. It is essential to accuracy that the tube 
be of even bore. Why? 

Mercury and alcohol are commonly used in liquid thermometers, partly 
because of their even expansion at all ordinary temperatures, and partly 
because of their low freezing points. Mercury freezes at — 40° F., and 
alcohol at about — 200° F. In the winter in high latitudes the temper- 
atures are too low to be recorded by mercury thermometers. On the 
other hand mercury is the better suited for high temperatures, since its 
boiling point is 660° F., while that of alcohol is only about 173° F., or 
lower than the boiling point of water. 

Maximum Thermometer. — It is often desirable to know the 
highest temperature attained during a given period. For this 
purpose the maximum thermometer is used. This is a modifica- 
tion of the ordinary liquid thermometer by a slight constriction in 
the bore just above the bulb. This narrowed bore, though wide 
enough to allow the expanding liquid to press through, is too nar- 
row for the liquid column, of its own weight, to pass back as the 
temperature falls. The thermometer thus continues to indicate 
the highest temperature attained. 

The clinical thermometer used by physicians is a maximum 
thermometer. To set the instrument for a new reading the col- 
umn of liquid must be made to unite by swinging or jarring the 
instrument. 

Minimum Thermometer. — The minimum thermometer, for reg- 
istering lowest temperatures, is simply an ordinary alcohol ther- 
mometer, with colorless liquid, containing a short double-headed 
pin. The heads of the pin are slightly smaller than the bore, in 
order that the alcohol may pass by the pin. 

For registering a minimum temperature the tube is placed in an in- 
clined position, so that gravity cannot pull the pin down the tube; but 



TEMPERATURE OF THE AIR 



79 



when gravity is assisted by the surface tension of the liquid, when the 
upper end of the contracting column comes in contact with the upper head 
of the pin, the pin is pulled down the tube. When, with rising temper- 
ature the liquid column begins to lengthen, it passes over and by the 
pin, but cannot push the pin against gravity up the tube. The upper 
end of the pin thus registers the lowest or minimum temperature at- 
tained. 

To set the instrument for registering a new minimum the thermometer 
is held, bulb upward, until the pin sinks through the liquid to the end of 
the column. The instrument is then placed in the inclined position in 
which it ordinarily rests. 

Thermograph. — To obtain a continuous record of the tempera- 
ture a self-registering thermometer, or thermograph is used. The 




Fig. 35. — Thermograph 



varying temperature is recorded by a pen, moved by a system 
of levers. The pen rests against a disc or cylinder of paper which 
is moved by clock-work. A continuous trace of the pen is made 




Fig. 36. — Thermograph Record for One Week 
Note daily variation of temperature, and hour of highest and lowest temperature. 

which, by reference to two sets of lines ruled upon the disc or 
sheet, temperature lines and time lines, shows the temperature at 



8o • PHYSIOGRAPHY 

any time. The thermograph takes the place of the maximum and 
minimum thermometers. 

The record made by the thermograph is a temperature curve for 
the period of time covered by the record. (See Fig. 36.) 

Approximately accurate temperature curves may be made from ob- 
servations of the thermometer taken every two hours. From the daily 
averages monthly curves, and from the monthly averages annual tem- 
perature curves may be constructed. 

Distribution of Insolation. — The amount of insolation received 
by a given area of land or water in a given time, as during one 
complete rotation of the earth, depends mainly upon the following 
variables: 

1. Length of insolation period, or the number of hours of sun- 
shine; 

2. The angle at which the insolation rays are received; 

3. Condition of the air as regards dust and cloudiness; 

4. Distance from the source of insolation, the sun; 

5. The length of the path of the rays through the atmos- 
phere. 

Length of Insolation Period. — Because the earth's axis is in- 
clined to the plane of its orbit the insolation period is not the 
same for all places, nor for the same place at all times. Most 
places upon the earth have the period of rotation unequally 
divided between sunshine and shadow. 

At the equator the period of insolation is always twelve hours. 
In all other latitudes it is only at the equinoxes that the insolation 
period is twelve hours; being longer than twelve hours when the 
sun is on the same side of the equator as the observer, and shorter 
than twelve hours when the sun and observer are on opposite 
sides of the equator. 

The higher the latitude the greater the length of the continuous 
insolation period. Within the polar circles it varies from no 
insolation in mid-winter to twenty-four hours of insolation in mid- 
summer, for each rotation. 



TEMPERATURE OF THE AIR 



fix 



Relation of Latitude to Greatest Length of Day or Night 



Latitude 


Greatest Length of 
Day or Night 


Latitude 


Greatest Length of 
Day or Night 


o° 


12 hrs. oo mins. 


5o° 


16 hrs. 04 mins. 


5° 


12 " 16 " 


55° 


17 " 00 " 


IO° 


12 " 4 " 


6o° 


18 " 15 " 


15° 


12 " 52 " 


65° 


20 " 48 " 


20° 


I 3 " 12 " 


66.5 


24 " 00 " 


25° 


13 " 34 " 


7o° 


64 days 


o 


-- « ~ . " 


- -° 


a 


3° 


J 3 54 


75 


io 3 


35° 


14 " 20 " 


8o° 


133 " 


40° 


14 " 48 " 


85° 


160 " 


45° 


IS " 20 " 


90° 


187 " 



Other things being equal the amount of insolation received varies 
as the length of the insolation period. There is, therefore, at 
summer solstice a constantly increasing amount of insolation, 
during one rotation, from the equator to the polar circle of the 
summer hemisphere; and a constantly decreasing amount from 
the equator to the polar circle of the winter hemisphere. 

Angle of Insolation. — Since the earth's shape is globular, the 
angle at which the sun's rays strike at any place varies with the 
latitude and with the time of day. This angle is zero at sunrise 
and sunset at any station, and is a maximum at noon. 

Because of the inclination of the earth's axis and revolution the 
angle of the sun's rays varies at any station from day to day. 
Vertical noon insolation occurs at the equator at the times of the 
equinoxes; and at the tropics, alternately, at the times of the 
solstices. During the year vertical noon insolation occurs twice 
at every station within the belt, forty-seven degrees wide, lying 
between the Tropics. This belt is sometimes called the torrid 
zone. No place outside this zone ever receives vertical insola- 
tion, the maximum angle being less and less with increase of 
latitude, reaching 23^° at the poles. 



82 



PHYSIOGRAPHY 



Hence, in so far as the angle of insolation determines the amount 
of insolation received during one rotation, the maximum amount is 
always received upon or between the Tropics. The average for the 
year is greatest at the equator and least at the poles. 




Fig. 37. — Showing Relation of Angle of Insolation to Intensity of Insolation 
Surface AB, which receives 100% of insolation when vertical, receives but 25.3% when the angle 

of insolation is 15°. 



Condition of the Air. — The two most variable constituents of 
the air are likewise those which most intercept insolation. These 
are, in the order of their importance, cloud-particles and dust. 
Clouds and dust in the air intercept insolation, and thus prevent 
land and water surfaces from being as much heated as they would 
otherwise be. For this reason those places where cloudiness pre- 
vails have a more constant temperature than places with prevail- 
ingly clear skies. Cloudy days are less warm in summer and less 
cold in winter than are clear days; and the insolation on the 
mountain top is more intense than in the valley. 



TEMPERATURE OF THE AIR 



83 



Distance From Sun. — The amount of insolation received varies 
inversely as the square of the earth's distance from the sun. While 
this factor has a scarcely perceptible value as between any two 
places upon the earth, at any given time, the difference in dis- 
tance being never as much as four thousand miles, yet as between 
winter and summer the value is considerable. The earth is about 
three million miles nearer the sun at perihelion, about January 
first, than at aphelion, about July first. In consequence a place 
receiving vertical insolation January first receives about 5% more 
insolation than one receiving vertical insolation July first. 

Length of Path Through Air. — Oblique rays pass through a 
greater thickness of air than do vertical rays; and whereas verti- 
cal rays lose half of their intensity, rays approaching tangency lose 
more than 90%. 

Intensity of Insolation at Different Angles 



Altitude of the 
Sun 


Relative Length of Path 


Intensity of Insolation 


Intensity of Insolation 


of Ray Through 


on Surface Perpen- 


on a Horizontal 


Atmosphere 


dicular to Rays 


Surface 


o° 


44.7O 


O.OO 


O.OO 


ro° 


5-7° 


O.31 


O.05 


20° 


2 .92 


0-5I 


O. 17 


3°° 


2.00 


O.62 


O.31 


40° 


156 


O.68 


O.44 


5o° 


131 


O. 72 


0.55 


6o° 


115 


°-75 


O.65 


7o° 


1 .06 


0.76 


O.72 


8o° 


1 .02 


0.77 


O.76 


oo° 


1 .00 


0.78 


O.78 



While the poles alternately receive more insolation than any 
other portion of the earth, for a brief period about the summer and 
winter solstices respectively, owing to continuous insolation there, 
all the conditions combine to give to places at the equator about 
two and one-half times the amount of insolation annually received 
at the poles. 



8 4 



PHYSIOGRAPHY 



Distribution of Heat Over the Earth. — The distribution of heat 
over the earth does not agree with the distribution of insolation, 
though in general following it, since the same factors govern the 
distribution of both. It should be remembered that heat is caused 
by absorbed insolation, and whatever factors enter into the control 
of absorption to that extent affect the temperature of the absorb- 
ing substance. 

Distribution of Insolation 









Latitude 








o° 


30° 


40° 


6o° 


8o° 


— go" 


Vernal equinox 

Summer solstice 

Autumnal equinox . . . 
Winter solstice 


1 .000 

0.881 

0.984 

0.942 


0.934 

1 .040 

0.938 

0.679 


O.763 
LI03 
0.760 
0.352 


0.499 
I.090 

0-499 
O.OS3 


O.OOO 
I.202 
O.OOO 
O.OOO 


O.OOO 
O.OOO 
O.OOO 
I.284 



Increasing obliquity of the sun's rays is accompanied by a more 
rapid decrease of heat developed than of insolation received. It 
results in an increased per cent of insolation reflected and conse- 
quently a decreased per cent absorbed. It is found that while water 
reflects only 2% of vertical insolation, it reflects about 65% when 
the sun is only ten degrees above the horizon. On this account 
the early morning and late afternoon rays, and the rays received 
in high latitudes, have little effect in increasing temperatures. For 
this reason alone the polar regions could never be warm; and the 
low minus temperatures reported by our Arctic and Antarctic 
explorers as occurring there in mid-summer are in part accounted 
for. 

When we consider also the fact that in the polar regions the lands 
and frozen seas are for much of the year covered with snow and ice, 
both very poor absorbers, and that the heat produced by absorp- 
tion must first be used to melt the ice-cap, we may better appre- 
ciate the low temperatures which prevail there. 

The northern hemisphere, where the continents are massed, is 
warmer in summer and colder in winter than the southern hemi- 



TEMPERATURE OF THE AIR 85 

sphere, which is mostly water. This is due to the fact that land 
is a better absorber and better radiator than water, and the fur- 
ther fact that it requires more heat to warm the water than the 
land. 

Dark colored rocks and soils, being better absorbers than light 
colored ones, are warmer under sunshine and colder when insola- 
tion is withdrawn. This, in a measure, controls the character and 
amount of plant growth, and affects the distribution of heat. 

The direction and character of winds and ocean currents, to be 
explained, are likewise important factors in the distribution of 
heat over the earth. 

All things combine to give to regions along the equator the 
greatest total amount of heat, and to make its distribution through 
the year most equable there. 

Shifting of Heat Equator. — This zone of greatest heat near the 
geographical equator, and of varying width, is known as the doldrum 
belt, or simply the doldrums. The line in the midst of this belt, 
passing through places having the highest temperatures, is called 
the heat equator. 

Since the sun's vertical ray shifts during the year over a zone 
forty-seven degrees wide, so the doldrums and heat equator shift, 
though over a narrower zone. 

The temperature of a place continues to increase so long as 
more heat is received than is lost by radiation. The change from 
warming up to cooling down occurs, during the day, ordinarily an 
hour or two past noon, though most heat is received at noon; and 
the highest temperature of the year occurs usually some weeks 
after the longest day, although most heat is received on that 
day. 

The doldrum belt and heat equator, therefore, do not attain 
their extreme positions north and south at the times of the sol- 
stices, but weeks after. Places between the Tropics, having 
vertical insolation twice a year, have two maxima and two minima 
during the year, and experience their highest maximum tempera- 
ture shortly after vertical insolation upon the sun's return toward 
the equator. 



86 



PHYSIOGRAPHY 



Average Position of Heat Equator. — The heat equator shifts far- 
ther, and remains for a longer time, north of the terrestrial equator than 
it does south of it. This is in part because the sun is seven days longer 
north of the equator than south of it; and in further part because of the 
forms of the continents and ocean basins. Owing to the positions and 
outlines of the continents more of the warm ocean currents are turned 
into the northern oceans than into the southern, and these make the 
northern hemisphere on an average the warmer. 

Moreover, the Pacific basin, being almost closed at the north, thus 
practically shutting out the cold polar currents that freely enter the 
North Atlantic, makes the North Pacific a warmer ocean than the North 
Atlantic. The average position of the heat equator is, therefore, more 
northerly in the Pacific than in the Atlantic. 

Shifting Most Over Atlantic. — Being a better absorber and better 
radiator than water, land has a higher temperature in summer and a 
lower temperature in winter than the sea in the same latitude. This 
excessive warming and cooling is most pronounced in its effects in the 
northern hemisphere, where the great land areas are; and. is also more 
pronounced over the relatively narrow Atlantic than over the broader 
Pacific. 

The accompanying table shows the approximate widths and extreme 
positions of the doldrum and trade wind belts during the year in both 
the Atlantic and Pacific oceans: 



N. E. Trades . 
Doldrums 
S. E. Trades . 



Atlantic Ocean 



March 



26° N- 3 N 
3°N-o° 
o° -25 S 



September 



3S°N-n°N 

n° N- 3°N 

3°N-2S°S 



Pacific Ocean 



March 



25° N- 5 N 
5° N- 3 N 
3° N-28 S 



September 



30 N-io° N 

io° N- 7 N 
7° N-2o° S 



Isotherms. — Lines drawn through places having the same tem- 
perature are called isotherms. They may represent the distribu- 
tion of temperatures at any given time, or they may represent the 
averages for any desired period, as a week, a month, or the entire 
year. Such lines, while very irregular, have in the main a general 
east-west direction. This is as we should expect, inasmuch as 
length of insolation period and angle of insolation, the most impor- 
tant factors in determining the distribution of heat, are constant 



TEMPERATURE OF THE AIR 87 

along any given parallel. The minor factors in the distribution of 
heat are responsible for the departure of isotherms from the 
parallels. 

Isotherms are continuous lines, and for a limited area may appear 
upon the map as closed curves. From their definition two isotherms 
cannot intersect. The heat equator is not an isotherm, though it extends 
around the earth in the same general direction as isotherms. It may 
cross isotherms. 

Temperature Gradient. — If we pass from one isotherm to the 
next of higher or lower temperature, we must pass through all 
intermediate temperatures. While we may pass along an indefi- 
nite number of routes, it is evident that the shorter the route the 
more rapid the change of temperature. The shortest route, which 
gives the maximum rate of change, is the direction of the tempera- 
ture gradient. 

Temperature gradient may be defined as: The rate of change of 
temperature measured in F. degrees, in a distance of one latitude 
degree, or about seventy miles. 

The more closely the isotherms are crowded the more rapid the 
change of temperature, or as we say, the steeper the gradient; while 
widely separated isotherms indicate gentle gradients. 

Isothermal Charts.— If the isotherms of any region be drawn 
the result is an isothermal chart. Daily, monthly, seasonal, and 
annual charts are commonly made. 

Isothermal charts are graphic representations of temperature 
readings where time is constant and place variable; whereas tem- 
perature curves are records with place constant and time variable. 

Vertical Distribution of Heat. — If we ascend through quiet air, 
as in a balloon, we shall find that, as a rule, the temperature of the 
air decreases', descending, the temperature increases. This change, 
due to difference of altitude, is about i° F. for every 300 feet, and 
is known as the vertical temperature gradient. 



PHYSIOGRAPHY 




TEMPERATURE OF THE AIR 



89 




90 PHYSIOGRAPHY 

QUESTIONS 

i. If the interior heat of the earth were the chief source of heat, what 
part of the earth's surface would be hottest? 

2. What reason have you for thinking that the sun, rather than some 
other outside source, is the chief source of our heat? 

3. What per cent of the sun's radiant energy is received by the earth? 
Does Mercury, Venus or the Earth receive the largest per cent ? 

4. Why are dark shades of clothing better suited to winter than to 
summer? Why are dark colored soils earlier ready for seeding than 
light colored? 

5. Why do we heat our kettles from below; and why place the radia- 
tors that heat our rooms near the floor rather than near the ceiling? 

6. Aviators find the air at the height of a few thousand feet always 
cold; why is this? 

7. The higher we ascend in the air the more intense the insolation; 
then why are the tops of high mountains always cold? 

8. Why do lakes and rivers cool down so much more quickly than they 
warm up? Why do shallow lakes freeze over more quickly than deep? 

9. Why is the temperature of Denver more equable than that of St. 
Louis? Chicago more equable than Minneapolis? 

10. Why is a uniform bore necessary in the tube of an accurate ther- 
mometer? Why is the tube expanded at the bottom? 

11. How can you use the thermometer to determine altitude? 

12. Why do flowers bloom and the trees put forth their leaves so much 
earlier on the south than upon the north slopes of mountains and hills? 

13. Why is a cloudy day in winter warmer, and in summer cooler, than 
a clear day? 

14. Why is it warmer in summer in the latitude of St. Louis than 
at the equator? 

15. Why is the warmest hour of the day later in summer than in 
winter, and why is the coldest hour earlier? 

16. Why is there less difference between the two maximum tempera- 
tures during the year than between the two minimum, at places over 
which the vertical ray of the sun shifts? 



CHAPTER IX 
WEIGHT AND DENSITY OF THE AIR 

Pressure and Weight. — It is a well-known fact that at any point 
within a liquid or a gas pressure is equal in all directions. On 
this account one moves freely about in the air, although it is press- 
ing upon every square inch of the body with a pressure of almost 
fifteen pounds, or more than a ton to the square foot. Neverthe- 
less this great pressure causes us no inconvenience, because it is 
balanced by an equal pressure from within. Pressure of the air 
is pressure per unit area. 

The pressure of the air sustains, at sea level, a vertical column of 
water about 34 feet high, and a vertical column of mercury about 
30 inches high. This fact is applied in the lifting pump, the 
siphon, and the barometer. 

A cubic foot of air at sea level weighs about 1.25 ounces. In a 
room 14 ft. long by 12 ft. wide by 10 ft. high there are more than 
125 pounds of air; and the weight of the air above an acre of ground 
is almost 50,000 tons. The weight of the air above any horizontal 
surface is equal to the pressure upon that surface, weight being 
simply pressure downward. 

Density of the Air. — In gases, pressure, density, and volume 
bear a definite relation to each other. As the pressure increases 
the density also increases, and the volume decreases in the same 
ratio. This is not true of liquids or solids. As a result of this 
relation the air is densest at the bottom. 

So rapidly does the density of the air decrease as we ascend in 
it, that at an altitude of about 3.6 miles the air is only half as 
dense as at sea level. This means that half of the air is within 
3.6 miles of the surface of the sea; and since many mountains are 
more than three miles high, their summits reach above one-half of 



92 



PHYSIOGRAPHY 



the entire mass of the air. Withing the next three miles we pass 
through almost one-half of the remaining half of the air; so that 
three-quarters of the air is within 6.8 miles of the surface of the 
sea. If the air were of the uniform density of the lower air, it 
would extend only about five miles above sea level. 

Measurement of Pressure. — For the purpose of measuring the 
pressure of the air the barometer has been devised. Its construc- 
tion depends upon the principle that a given weight of air will 
balance an equal weight of any other fluid; or counterbalance an 
equal pressure exerted in any other way. 

Two types of barometer are in common use, the liquid and 
non-liquid. In the liquid barometer the air sustains a column of 
liquid, commonly mercury, in a tube from which the air has been 
withdrawn. In the non-liquid or aneroid barometer, the pressure 
of the air is counterbalanced by the resistance of a thin metal 
diaphragm. 

The simple mercury barometer consists essentially 
of a glass tube at least thirty-four inches long, closed 
at one end, filled with mercury and placed vertically, 
open end down, in a cistern of mercury. The tube is 
graduated in some linear unit, as the millimeter or 
tenth of an inch, the surface of the mercury in the cis- 
tern being the zero of the scale. 

The mercury sinks in the tube, leaving a few inches 
of the upper end of the tube a vacuum, that is, with 
no air pressure on the mercury column. The column 
of mercury is sustained by the pressure of the air upon 
the open surface of mercury in the cistern. When this 
pressure increases the mercury rises in the tube, and 
when it decreases the mercury sinks. 

At sea level the length of the mercury column is 
about thirty inches; hence we commonly say the air 
pressure at sea level is thirty inches, understanding 
that the pressure is measured by the weight of the col- 
umn of this length, as pressure cannot be measured 
in inches. 



WEIGHT AND DENSITY OF THE AIR 



93 




As it is the vertical length of the column of mercury that 
measures the pressure of the air, it is necessary, when tak- 
ing a reading, to hold the instrument in a vertical position. 
For this purpose, and to protect the instrument, the tube 
and cistern are firmly bound together to a rigid frame, 
arranged for suspension. 

The aneroid barometer consists essentially of a pile 
of hollow metallic discs, from which the air is ex- 
hausted, and to which an index is attached. This 
index moves over a surface upon which there are 
graduations to represent the various pressures. The 
discs are made of very thin metal, supported by coiled 
springs within, and respond to slight changes in air 
pressure. Because of its convenience the aneroid is 
much used in taking altitudes. Both pressure in 
inches and altitudes in feet are usually shown. 

Variation in Barometer Reading. — At sea level, as we 
have seen, the average reading of the barometer is 
about thirty inches. As the instrument is carried up 
through the air, in a balloon or in ascending a moun- 
tain, it is found that the barometer reading is lower 
by about one inch for each thousand feet of ascent. 
This is because of the air that is left below, only the dardIbarometer 
air above affecting the barometer. 

This is only approximately true, for with increase in altitude there is 
a decrease in the density of the air. Whereas a fall of one inch results 
from carrying the instrument from sea level up 910 
ft., a fall of two inches requires an ascent of 1 ,850 ft. 
The higher the altitude the greater the distance 
through which the instrument must be carried to 
register a fall of one inch. 

Height of the Air. — Estimates of the thick- 
ness of the air envelope, based upon barometer 
readings, are unreliable, inasmuch as we do not 
know at what rate the density of the upper 
air changes. While one-half of the air lies 
within 3.6 miles above sea level, we have reason 




Pig. 42. — 
Aneroid Barometer 



94 PHYSIOGRAPHY 

to know that the air in considerable density exists at a height 
of about two hundred miles. Meteors have been observed at 
that height. 

Lows and Highs. — If a stationary barometer be read from hour 
to hour it will be noted that its readings vary continuously. This 
seems to be due chiefly to a succession of surges in the air, called 
lows or highs as the barometer falls or rises. Lows are also called 
cyclones and highs anti-cyclones. 

As we go outward from the center of a low we observe that the 
barometer readings are higher in all directions; and in passing out 
from the center of a high the readings are lower. It follows that 
about lows and highs systems of lines may be drawn through 
places having the same barometer reading. Lines drawn through 
places having the same barometer reading are called isobars. 

Because of the mobility of the air, and the many conditions 
that affect pressure, isobars are not apt to be either regular or 
parallel, although about lows and highs that are strongly devel- 
oped isobars are closed curves. 

The isobars about a high may be aptly likened to the contours of 
a hill in a topographic map, the high by analogy being an atmos- 
pheric kill; and those about a low to the contours of a depression, 
the low being an atmospheric hollow. 

Inasmuch as the density varies directly as the pressure, the air 
is denser about a high than about a low. 

Pressure Gradient. — Just as the temperature gradient line is 
the shortest distance from one isotherm to the next, so we may 
get the pressure gradient line at any place by taking the shortest 
distance between the isobars at that place. Numerically expressed, 
the pressure gradient is the number of hundredths of an inch 
change of pressure, in a distance equal to one latitude degree, or 
about seventy miles. Crowded isobars, therefore, signify steep 
pressure gradients, and widespread isobars- gentle gradients. We 
shall see that the direction and strength of the wind are closely 
related to the pressure gradient. 

If a continuous record of the air pressure is desired, an instru- 
ment called the barograph is used. It is usually an aneroid ba- 



WEIGHT AND DENSITY OF THE AIR 



95 



rometer with a pen-bearing arm in the place of the index. The 
pen-point rests against a disc or sheet of paper that moves at a 
constant rate, as in the thermograph. The two systems of lines 
are time and pressure lines. 

The record of the barograph is a pressure curve, and the charted 
pressures of any region, as shown by 
the isobars, make an isobaric or pres- 
sure chart. 

Pressure Belts. — The distribution 
of pressure over the earth is intimately 
associated with the distribution of 
heat ; and as the equatorial regions are 
regions of high temperature, they are, 

as a result, regions of low pressure. The air being excessively 
heated, is pushed away from over these regions, leaving them 
deficient in pressure. On either side, in the region of the Tropics, 
the pressure is increased, thus giving a high pressure belt in 
each hemisphere. 

Poleward from the tropical high pressure belts the pressure, as 




Fig. 43. — Barograph 




Fig. 44. — Barograph and Thermograph Records for One Week 
Note their Relation. As the barometer rose the thermometer fell, and as the barometer 
the thermometer rose. Daily variation of temperature obscured by the variation due 
the passing high and low pressure areas. 



Ml 
to 



9 6 



PHYSIOGRAPHY 




WEIGHT AND DENSITY OF THE AIR 



97 




98 PHYSIOGRAPHY 

a rule, decreases; and the polar areas are thought to be relatively 
low pressure areas. 

As a result of this arrangement of pressure, the isobars of the 
world have a general east-west trend, and shift with the shifting 
belt of equatorial heat. 

Uses of the Barometer. — As before stated, the aneroid barom- 
eter is used in the determination of altitudes, because of its con- 
venience in carrying. Aviators and balloonists carry aneroids, 
this being often the only means by which the altitude reached by 
them can be known, as they are obscured by the clouds from the 
view of observers upon the land. 

But a much more important use of the barometer is in fore- 
casting the weather. Pressure is one of the factors which deter- 
mine the weather; and in forecasting the weather a knowledge of 
the distribution of pressure over the country is necessary. 

QUESTIONS 

i. In a closed vessel, filled with air, the pressure upon the inner sur- 
face decreases with decrease of temperature, whereas in the open air 
pressure increases with decrease of temperature; why is this? 

2. What is meant when we say the pressure of the air is 30 inches? 

3. Why does the air at any place vary in pressure? 

4. How may the barometer be used to measure altitude? 

5. Why must a liquid barometer be held in a vertical position when 
read? Why is it not necessary to hold an aneroid in a definite position? 

6. Why is it not necessary that the bore of the barometer tube be 
regular as in the thermometer? 

7. What is the general relation between barometer change and change 
of thermometer? 

8. Why is mercury so generally used in the construction of liquid 
barometers? What is the objection to using water? 

9. Why do standard barometers have a thermometer attached? 
10. Why do the high pressure belts of the " horse " latitudes shift? 



CHAPTER X 
MOVEMENTS OF THE AIR 



Winds Denned and Explained. — The air, being part of the 
earth, by necessity partakes of the earth's motions of rotation 
and revolution. Entirely distinct from these motions are those 
sometimes regular, but more often fitful and irregular movements 
of the lower air, called winds. 

A wind may be defined as an approximately horizontal natural 
movement of the lower air. Winds should be sharply distinguished 



Kr- 

— !«- 



-iL 



7N~ 



"A" 



"7K" 



(30-5) 



-> 



(2-9) 
HEAT 



<r — 



(30-5) 



30 



30 



30 

Fig. 47.— Showing How Winds are Started 

Numbers below A B indicate barometer reading before heating; those above A B barometer 

reading after heating. 

from vertically moving air; likewise from the upper-air movements, 
both of which are called currents. 

The air is so mobile that any object moving through it sets a 
considerable volume in motion; and the least inequality of pres- 
sure disturbs its equilibrium. The most important cause of the 
unequal distribution of pressure, and therefore of winds, is the 
unequal distribution of heat over the earth. 



IOO PHYSIOGRAPHY 

Figure 47 and its explanation are easily applicable to earth 
conditions, and to the explanation of winds. 

A B represents any surface, above which the air extends to the height 
G H. Suppose a limited area, as C D, is heated in excess of the surround- 
ing surface. The air above C D expands, and if we consider only the 
expansion upward, the column of air above C D lengthens to K L. 
Being unsupported laterally the air in the column above the level of G H 
flows away until the upper surface of the air is again level at the height 
E F. Some air above C D having flowed away, the pressure upon this 
surface is less than before heating; whereas the pressure upon A C and 
B D is greater. , 

As a result of the rearrangement of pressures, A C and B D are 
highs, away from which the lower air moves laterally; and since C D 
is a low the movement is more pronounced toward this area. Above 
C D the inflowing currents meet and rise, thus completing the circulation. 
The circulation continues just so long as C D is heated in excess of the 
surrounding areas. 

If we consider the separate steps in the development of the circulation 
described they may be stated as: 

1. Local excessive heating; 

2. Expansion of the column of air above the heated area; 

3. Overflow aloft from, and inflow at the bottom toward, the heated 
area; 

4. Ascent of the inflowing currents above the heated area, or low; 

5. Descent of the outflowing currents. 

It is only the surface movement of this circulation that is called 
winds; all other parts are currents. 

Terrestrial Winds. — If we consider the foregoing figure a verti- 
cal section of the air along a meridian at the equator, we have an 
explanation of the systematic winds of the earth. 

The area C D represents the doldrum belt along or near the 
equator, which, by reason of vertical insolation, is most heated. 
The movement of the air above this belt being chiefly upward, this 
is a belt of light winds, or calms. 

The poleward overflow aloft leaves this region a belt of low 
pressure, and at the same time produces on either side, somewhere 
between the latitudes of 25 and 35 , a belt of high pressure. 
Above these belts of high pressure the movement of the air is 
chiefly downward, and these, like the doldrum belt, are also belts 



MOVEMENTS OF THE AIR 



IOI 




of light winds or calms. They are known as the Horse Latitudes. 
Out from these high pressure belts the winds blow toward the 
equator, and with less pronounced strength toward the poles. 

Excessive heating along a belt near the equator results from: 
(i) The globular shape of the earth; (2) Rotation about an axis 
which remains parallel to 
itself; (3) The source of heat 
being a body distant from 
the earth. 

Since most of the planets 
agree in these three par- 
ticulars, it follows that the 
circulation described for the 
earth must be common to all 
planets with atmospheres. 
On this account the winds 
produced by this circulation 
are sometimes called planet- 
ary winds. 

Deflection of Winds. — It 
was long ago discovered that 
winds do not follow a straight 
course; but in the northern 
hemisphere turn to the right, 
and in the southern hemi- 
sphere to the left of such a 
course. The statement of 
this systematic deflection is known as Ferrel's Law, and all 
winds obey this law. It governs alike the constant winds that 
blow out from the belts of high pressure calms, and the irreg- 
ular winds that blow about high and low pressure areas. Because 
of this deflection winds do not follow the barometric gradient. 

Deflection of winds from a straight course results from the rota- 
tion of the earth. 

In explaining the relation between deflection of the winds and rotation, 
two facts are important: 



vanuary 
West-jwrth-wtsterlies 




Fic. 48. — Terrestrial Wind Belts 
Owing to the inclination of the earth's axis, and 
revolution of earth around the sun, the wind 
bells shift. The figure shows their extreme 
northern and southern positions. Where they 
overlap we have the monsoons of the sub-tropical 
and sub-equatorial belts. Note the bending of 
the trades where they run across the equator, to 
be explained later. 



I02 PHYSIOGRAPHY 

i. The rotational velocity of places upon the earth decreases poleward; 

2. The inertia of matter makes it impossible for a particle once set in 
motion, of itself, to change its direction or motion. 

In I in the figure below, if a marble be started from the center along 
the radius C A, with sufficient velocity to carry it to the edge of the table 
in one second, if the table be at rest the marble will leave the table at A. 
If, however, at the instant the marble is set in motion the table be set 
rotating in a counter-clockwise direction at such rate as to change the 





I II 

Fig. 49 .■ — Illustrating the Deflection of the Winds from a Straight Course, Due to 
the Rotation of the Earth 



position of radius C A to that of C A' in one second, the marble will fol- 
low the curved path, leaving the table at B. The marble's inertia has 
made it take the curved path, deflected continuously to the right of a 
straight course as it moved into regions of greater rotational velocity. 

If instead, the marble be started from A along A C at the. instant the 
table is set rotating, in one second it will have traversed the path A' B'. 
The inertia of the marble has again made it take a curved path, deflected 
to the right of a straight course, as it moved into regions of less rotational 
velocity. 

On any straight line that may be drawn upon the table, along which 
the marble may be started, the marble will either approach the center or 
recede from it; and since in both cases the^ counter-clockwise direction 
of rotation causes a right-handed deflection, the marble will in all cases 
be deflected to the right of the straight course along which it is started. 

The rotation of the earth, as seen from above the north pole, is counter- 
clockwise; and winds in the northern hemisphere behave in an analogous 
way to that of the marble described above in I. 

As seen from above the south pole the earth's rotation is clockwise, 
and deflection is to the left of a straight course, as shown in II. 



MOVEMENTS OF THE AIR 103 

The deflective effect of rotation may be illustrated by pouring water 
on a rotating globe. If the globe be held with its axis vertical, and 
rotated in a direction corresponding to that of the earth, the minute 
streams will be deflected to the right of the meridians along which they 
start so long as they are in the northern hemisphere, and to the left after 
they cross the equator. If started from the south pole of the globe they 
will suffer first left-handed deflection, changing to right-handed upon 
crossing the equator into the northern hemisphere. 

The only winds which do not suffer deflection are those along the 
equator. Set moving in any other direction or in any other latitude 
they suffer deflection according to Ferrel's Law. The deflective effect 
of rotation increases with the latitude, and in any latitude varies in- 
versely as the velocity of the wind. It should be remembered that the 
rotation of the earth has no power to set the air in motion, and its deflective 
power exists only after motion is started. 

Two important laws governing winds are: 

1. Winds always blow from a region of higher pressure to a region 
of lower pressure, with a velocity which varies with the pressure 
gradient; 

2. On account of the rotation of the earth winds turn to the right of 
the pressure gradient in the northern hemisphere, and to the left of 
the pressure gradient in the southern hemisphere. 

Description of Wind Belts. — The wind belts depend upon the 
pressure belts, and these in turn are determined by the distribu- 
tion of heat. Since the heat belts shift, in like manner the pres- 
sure and wind belts shift. 

The Doldrums are so named because of the light winds and 
calms that characterize this belt. It is a belt of high tempera- 
ture, and consequently low pressure. The winds move obliquely 
in toward the doldrums from both sides. The low pressure is of 
convectional origin, and the rapid ascent and cooling of the air 
give rise to frequent and abundant rains. 

The Trades are the winds that blow from either side obliquely 
in toward the doldrum belt. They derive their name from their 
constancy of duration and direction, qualities important to sailing 
vessels engaged in trade. In the northern hemisphere they are 
called northeast trades, and in the southern, southeast trades. They 
have their origin in the high pressure belts of the horse latitudes, 



104 PHYSIOGRAPHY 

and move toward the low pressure belt of the doldrums. Instead 
of following the steepest gradients, along the meridians, as they 
should do if there was no rotation, they are deflected, in accord- 
ance with Ferrel's Law, to the right in the northern hemisphere, 
and to the left in the southern. 

In order to reach the doldrum belt the trade-winds sometimes 
have to cross the equator. When northeast trades cross to the 
south of the equator they become northwest winds; whereas south- 
east trades crossing the equator become southwest winds. These 
"hooked" winds are due to the difference in the deflective influence 
of the earth's rotation north and south of the equator. 

The Horse Latitudes are belts of high pressure next to and pole- 
ward from the trades. The air that is warmed, and by convection 
rises in the doldrums, moves away poleward at high altitudes. 
Its overflow aloft, and consequent piling up in the regions of the 
horse latitudes, causes the high pressure and descending currents in 
these belts. As a result of its descent the air is warmed by com- 
pression, and its capacity for water vapor is increased. These 
belts are therefore prevailingly dry. The trade-winds begin 
here; likewise the less regular winds that move away toward the 
poles. 

The Prevailing Westerlies flow away from the horse latitude 
belts toward the poles, being deflected to the east by the rotation 
of the earth. They are named from their direction, and in their 
upper parts are continuations of the overflow above the trades. 
The prevailing westerlies are neither so constant in duration nor 
in direction as the trades. 

As the prevailing westerlies approach the poles obliquely and 
along converging courses, in each hemisphere there is developed a 
circumpolar whirl. These winds spiral about the north polar re- 
gion in a left-handed or counter-clockwise direction, and about 
the south polar region in a right-handed or clockwise direction. 

As a result of the spiral inflow toward the poles it is believed the polar 
regions are areas of low pressure. The centrifugal force resulting from 
the whirl tends to heap the air out around the polar center, and to pro- 
duce a circumpolar ring of high pressure. From this ring the winds 



MOVEMENTS OE THE AIR 105 

move away into lower latitudes. The frequent northeast winds ob- 
served on the northern coast of Alaska are thus explained. 

Both temperature gradient and pressure gradient, between the 
equator and the Arctic regions are steeper in winter than in summer. 
As a consequence the circumpolar winds are strongest in winter. 
This may account for the frequency and violence of our winter 
cyclones. 

Because of the excessive cooling of the northern continents in 
winter, the North Atlantic and North Pacific oceans are warmer 
than the northern continents and are therefore centers of low 
pressure. About these centers the winds spiral in a way com- 
parable to the circumpolar whirl ; and from these secondary centers 
winter cyclones are projected. Those from the Pacific often move 
southeastward into Canada and the United States. 

Shifting of the Wind Belts. — The pressure belts and wind belts 
follow the shifting of the belt of greatest heat. As a result of this 
shifting, places near the border of the various wind belts lie alter- 
nately in two belts. Southern Florida, southern California and 
northern Mexico lie alternately in the trades and the horse lati- 
tudes; and the Amazon Valley, usually in the doldrums, is peri- 
odically swept over by the trades. 

This shifting of the wind belts has the effect of widening the 
belts which at some time during the year lie under the low pressure 
and the high pressure calms. To these widened belts the names 
sub-equatorial and sub-tropical are respectively given. 

Places so situated as to have seasonal change of wind are said 
to have monsoon winds, or simply monsoons. While many places, 
both in the sub-tropical and sub-equatorial belts have monsoon 
winds, perhaps the most pronounced and best known monsoons 
are those of the northern Indian Ocean, and the adjacent lands to 
the north and east. 

During the northern summer the heat equator migrates far into 
the heated continent of Asia. Then the southeast trades, which 
run to the doldrum belt, cross the geographical equator, and in the 
northern hemisphere become, according to Ferrel's Law, south- 
west winds. In winter the heat equator migrates southward, and 



io6 



PHYSIOGRAPHY 



over southern Asia and the northern Indian Ocean the strength- 
ened northeast trades blow. 

Continental Air Drifts. — The land has a higher temperature 
than the sea in the same latitude in summer, and a lower tem- 
perature in winter. As a result the continents are areas of low 




Fig. so. — Winds of Northern Indian Ocean for July 



pressure in summer, and of high pressure in winter. Following 
this adjustment of pressure there is a general outward drift of 
the lower air from the continents in winter, and an inward drift 
toward the continents in summer. 

These movements, which in a strict sense are monsoons, are 
never so called, but are sometimes called continental winds. They 
are so easily obscured by other winds as to be scarcely noticeable 
of themselves; and their chief office is to modify other winds. 
Thus upon our western coast the westerlies are weakened in winter 
and strengthened in summer; whereas upon our eastern coast they 
are strengthened in winter and weakened in summer. 



MOVEMENTS OF THE AIR 



107 



Land and Sea Breezes. — The winds that blow alternately from 
and to the land, at and near the seashore, are called land and sea 
breezes. When the land is colder than the sea the breeze blows 




Fig. 51.— Winds of Northern Indian Ocean for January 



from the land and is called the land breeze; and when the land is 
warmer than the sea the breeze blows from the sea, and is called 
sea breeze. The land breeze blows during the night, as the land 
is then an area of relatively high pressure; and the sea breeze 
blows during the day, the land being then a region of relatively 
low pressure. 

Advantage is taken of this twice daily change of breeze by fish- 
ing and pleasure craft that depend upon sails to carry their fleets 
away from and to bring them back to land. Similar land and lake 
breezes are felt along the shores of our great lakes and inland seas. 

If the land should remain throughout the twenty-four hours colder 
than the sea, then there would be a continuous land breeze. This often 
occurs in winter, especially when the land is covered with snow. On the 



108 PHYSIOGRAPHY 

other hand, in mid-summer it sometimes happens that the land does not 
cool down during the night below the temperature of the adjacent sea; 
then the sea breeze continues throughout the night. 

Mountain and Valley Breezes. — Another similar change of breeze 
may be noted upon the slopes of mountains or of deep valleys. The top 
of the mountain, or of the dividing ridge, being first warmed by the rays 
of the rising sun, convectional ascent begins there. As the slopes and 
thcTair in contact with them are_ warmed to lower and lower levels, the 
warmed lighter air finds an easier escape obliquely up the slope and 
through the convectional chimney above the summit than vertically 
through the overlying stratum of inert air. This obliquely ascending 
current is felt upon the slope as a valley breeze. 

At night radiation is most rapid from the mountain summit or ridge 
crest, and the cooled air from the upper slopes, being heavier, moves 
down the slopes toward the valley. This is the mountain breeze. The 
alternation of mountain and valley breezes may be interrupted in the 
same way as described for land and sea breezes. 

Avalanche winds occur in valleys between steep mountain slopes, and 
are caused by the moving mass of snow and rock on the mountain side. 
Being compressed in the narrow valleys, these winds sweep with almost 
irresistible force down the valley. 

Cyclonic Winds. — In temperate latitudes by far the most im- 
portant winds are those unperiodic winds, caused by the irregular 
distribution of pressure in lows and highs, and known as cyclonic 
winds. These lows and highs are best understood if considered as 
local disturbances in the terrestrial winds. Thus considered, their 
general drift with the winds of the belt in which they occur is 
accounted for. 

Cyclonic winds move obliquely in toward the center of a low, 
spiraling about the center in a counter-clockwise direction in the 
northern hemisphere, and in a clockwise direction in the southern 
hemisphere. They move spirally out from a high, turning in a 
clockwise direction in the northern hemisphere and in a counter- 
clockwise direction in the southern. 

Although their origin is obscurely understood, some lows are known 
to be of convectional origin; others are probably not. This has given 
rise to two classes of cyclones, convectional and non-convectional. 

Land varies indefinitely in its power to absorb insolation, owing to its 
almost infinite variety of composition, covering, and form. \i any given 



MOVEMENTS OF THE AIR 



109 



limited area is heated in excess of the surrounding region, the air above 
this area expands, overflows aloft, and the area becomes a low, while the 
surrounding region has increased pressure. The overflow from adjacent 
lows may unite and produce a high. On the other hand, the air over a 
given area, as a snow-covered region, may be excessively cooled and con- 
densed. Then the upper air from the surrounding region flows in upon 
it, producing a high, surrounded by a ring of lower pressure. Such lows 
and highs are of convectional origin. 

It seems probable that non-convectional cyclones originate in two 
ways. In high latitudes, about the margins of the circumpolar whirls, 
and in the northern hemisphere about the margins of the North Atlantic 
and North Pacific eddies, smaller eddies of air are developed; analogous 
to the eddies that arise about the perimeter of a strong water whirl. 
These are sometimes called driven cyclones, and are most frequent and 
best developed in winter when these great eddies are strongest. In 
lower latitudes, even within the tropics, cyclonic whirls may result from 
the friction between great poleward moving and equatorward moving 
masses of air. Such cyclones are called jrictional cyclones. They may 
originate in the higher currents, and sink to the bottom of the air fully 
developed. 

Movements of Lows. — Three distinct movements of the air 
with reference to lows must be distinguished: the spiral inflow 
of the wind toward the center of the low, ascent at the center, 
and the forward movement of the low itself. 

Wherever their place of origin, most lows finish their course in 
the zone of westerlies, following the general direction of these 
winds. In the United States three general paths are followed, as 
the cyclone originates in the northwest, in the southwest, or in the 
southeast within the tropics. Those originating in the northwest 
move first southeasterly, until they reach the axis of the Missis- 
sippi Valley, when they change to a northeasterly course for the 
remainder of their journey across the continent. Those having 
their origin in the southwest move systematically northeastward 
across the continent. Tropical cyclones having their origin in the 
region of the West Indies move first northwestward, until they 
reach the high pressure calms, north of which they conform to 
the course of the westerlies. These usually cross the high pressure 
belt over or near the land, the belt being less well developed 
there. 



HO PHYSIOGRAPHY 

All cyclones, sooner or later, conform to the course of the west- 
erlies. The upper air currents in all latitudes move to the north- 
east; and it is probable that the spirally ascending column of air 
at the cyclone center reaches to these upper currents. 

Strength of Cyclonic Winds. — The velocity of the wind increases 
as it approaches the center of a low; and if a strong spiral move- 
ment is developed the velocity is greatly increased. On the other 
hand, the winds start from the center of the high, and are therefore 
weakest there. Consequently, the strength of the wind increases 
with the approach of a low, and decreases with the approach of a 
high. 

Because of their greater strength, cyclonic winds obscure all 
other winds in regions where they occur. The winds poleward 
from the horse latitudes are chiefly cyclonic. 

When the low pressure area is very small the spiraling winds 
may attain excessive velocity, and become destructive. Such 
wind storms are known as tornadoes, and in their greatest strength 
the winds are so strong as to carry away the instruments for their 
measurement. Velocities of more than one hundred miles an hour 
have been measured. Similar storms in the West Indies are called 
hurricanes, and in the East Indies typhoons. 

Cyclones within the tropics, of wide extent, sometimes have 
within the whirl of destructive winds an area of clear skies. This 
area, called the " eye of the storm," may be as much as one-tenth 
of the diameter of the cyclonic area. Vessels passing through the 
eye of the storm experience equally strong winds in the front and 
in the rear of the cyclone, though from opposite directions. 

Shifting of Winds. — When a station lies near the path of a low 
or high the winds at the station shift in a systematic way as the 
barometric disturbance passes. In the westerlies of the northern 
hemisphere, where the disturbances advance from west to east: 

i. If a low passes north of the station, the first effect is to induce 
easterly winds. These veer (change with the sun) through the 
southeast, south, and southwest to some westerly quarter as the 
disturbance advances easterly. 



MOVEMENTS OF THE AIR 



ill 



2. If a low passes south of the station, the winds, northeasterly at 
first, back (change against the sun) through the north, northwest 
and west to a southwesterly direction. 




Fig. 52— A B is the path of the center of a low which passed north of the station S. The 
successive winds are numbered in order. The curved arrow X Y is the composite of the 
sheaf of arrows passing through S toward the center of the low. 



3. If a high passes north of the station, the winds, at first westerly 
or northwesterly, veer to northerly and northeasterly winds in 
succession. 




Fig. S3- — The path of the center, A B, is here south of the station S. As above, the curved 
arrow X Y is the composite of the sheaf of arrows through S. 



112 



PHYSIOGRAPHY 



4. If a high passes south of the station, the wind backs from the 
southwest through the south, southeast, and east in turn. 




Fig. 54. — In Fig. 53 a high passes north of the station S, and the succession of winds, 
blowing out from the center of the high, is shown by the sheaf of arrows through S. The 
curved arrow X Y shows their composite. 




Fig. 55. — The path of the center of the high is south of S, and the sheaf of arrows and their 
composite illustrate the backing of the wind. 



MOVEMENTS OF THE AIR 113 

Should the disturbance pass centrally over the station the wind 
will hold steadily as a northeasterly wind if the disturbance is a 
low, or as a westerly wind if the disturbance is a high, as the dis- 
turbance advances, changing suddenly through a calm to the 
opposite point of the compass as the center of the disturbance 
passes. 

In any other wind belt the direction of shifting may be determined by 
noting the direction in which the disturbance advances, and considering 
the direction of movement of winds about highs and lows in that belt. 
For any given station the direction of shifting is systematic and uniform 
for a given set of conditions. 

Special Winds. — In every part of the world winds of special 
character and of exceptional occurrence are known, to which local 
names are given. Some of these are warm winds, others are cold; 
some owe their character to change in latitude, others owe theirs 
to a change in altitude. 

The change in temperature due to change in latitude is, on an 
average, about i° Fahrenheit for each degree of latitude, whereas 
the temperature change in ascending currents of air is about i.8° F. 
for 300 feet. Therefore, any transfer of air, either in latitude or 
altitude, is accompanied by a change in temperature, both of the 
transferred air and of the region to which it is transferred. 

Generally speaking, winds blowing from lower to higher lati- 
tudes are warm winds, while those blowing from higher to lower 
latitudes are cool. Winds descending from higher to lower alti- 
tudes are generally cool or cold winds, though their temperature 
rapidly rises as they descend and makes them warm, while those 
blowing up a slope are apt to be warm. 

Among warm winds may be mentioned: 

The Hot Wave, of western central United States. It blows in 
summer from the west or southwest, sometimes continuing for 
days, and withers all growing vegetation. 

The Sirocco, a south wind from the Sahara desert, felt as far as the 
north shore of the Mediterranean Sea. It is commonly dry and dust 
laden. 

The Simoom, an intensely hot, dry, and generally sand-laden wind of 



ii4 



PHYSIOGRAPHY 



the Arabian desert. It is probably a convectional whirlwind, similar to 
the dust-laden whirlwinds of all dry, hot climates. It lasts usually less 
than ten minutes, and often forms sand-spouts. 

The Chinook, an American wind, which moves down the slopes 
of mountains toward a low pressure area at their base. Though 
starting upon its descent as a cold wind, it warms by compression 
in its descent, and if the mountain is high it may reach the base 
as a warm or even hot wind. In all cases, because of its dryness, it 
evaporates or melts the snow fields over which it blows, and often 
causes destructive avalanches, by melting the snow on the steep 
slopes. It is of frequent occurrence along the eastern bases of the 
Rocky and Sierra Nevada mountains. Many valleys here are 
kept practically free from snow; and their temperatures are so 
mild as to make shelter for stock in winter unnecessary, and to 
permit grazing throughout the year. The Chinook is scarcely 
noticeable in summer. 

The Foehn is the European wind of the Chinook type, common on 
the northern slopes of the Alps, where, in the north-south valleys it 
hastens the ripening of grapes in the fall; and in winter rapidly melts 
the snows in its path. This has earned for it the name of "snow-eater." 

To the class of cold winds belong: 

The Norther, of southwestern United States. It is the cold 
inflow of winds from the north, at the rear of the winter cyclone. 

A fall of temperature of fifty degrees 
in two hours has been noted. 

These winds often cause great suf- 
fering, loss of live stock, and even loss 
of human life. 

The Blizzard, the American name 
for a cold wind of high velocity, ac- 
companied by snow. It is common 
east of the Rocky Mountains, and 
often causes great loss of life among 
stock. 

The Bora and Mistral are European 

Fig. 56. — Anemometer cold winds. 




MOVEMENTS OF THE AIR n 5 

Velocity of Winds. — Winds are sometimes classified according 
to their velocity. The velocity of the wind is measured by means 
of the anemometer, and is expressed in miles per hour, or feet per 
second. 

When the wind reaches a velocity of ioo miles or more per hour 
the instruments are usually carried away. Therefore, we have no 
record of the velocity of the wind in our most violent tornadoes. 

The accompanying table gives the ordinary names for winds, 
their approximate velocities in miles per hour, and a common and 
easy way of judging them: 

Name Distinctive Characters Velocity 

Calm Flags limp, leaves unmoved o 

Light breeze .... Moves leaves of trees 1-5 

Fresh wind Moves branches of trees, blows up dust 5-15 

Brisk to strong. .Sways branches of trees, makes white caps 15-25 

High wind Sways trees, moves twigs on ground 25-35 

Gale Breaks branches of trees, dangerous for sailing..35-75 

Hurricane wind.. Destroys houses, uproots trees 75-100 

QUESTIONS 

1. Why does the wind blow? Why is there not always wind? 

2. What determines the direction and velocity of the wind? 

3. Interpret "no wind" in terms of pressure gradient. 

4. Why are summer days, as a rule, more apt to be windy than sum- 
mer nights? Than winter days? 

5. Why are the " trades," on the whole, stronger than the " wes- 
terlies? " 

6. Why are upper currents stronger than surface winds; and winter 
winds stronger than summer? 

7. Why are the " trades " more regular over the sea than over the 
land? 

8. Why are the upper currents in all latitudes westerly currents? 

9. Why do winds spiral about " lows ? " Why are not equally strong 
whirls developed about "highs?" 

10. Why are southern California and southern Florida more apt to 
have westerly winds in winter than in summer? 

n. Why are storms that come from the southwest often called "north- 
easters"? 

12. Why do cyclones in the eastern United States move so generally 
toward the northeast? 



Il6 PHYSIOGRAPHY 

13. Why do thunderstorms occur so rarely at night? 

14. What should be the direction of shifting of the wind with passing 
" lows " and " highs " at Havana, Cuba? At Buenos Aires? 

15. Why, in the natural ventilation of our rooms, do we admit the air 
at the bottom of the room, whereas in forced ventilation the air is ad- 
mitted at the top? Which is better? 

16. Why is a room better ventilated when windows are both raised 
from the bottom and lowered from the top than when merely raised or 
lowered? 

17. Why do fires burn better in cold than in warm weather? 

18. Why is a flue less liable to smoke after the fire is well started than 
when it is first lighted? 

19. Why do snow-drifts and sand-dunes accumulate behind an ob- 
stacle rather than in front of it? 

20. Why do aviators avoid flights over cities and deep canyons? 



CHAPTER XI 

MOISTURE OF THE AIR 

How Obtained. — The moisture of the air is obtained by evapora- 
tion from moist surfaces. Evaporation is the process by which 
solids and liquids are changed to vapor. While evaporation is chiefly 
from water surfaces, yet scarcely any land surface is so dry that it 
does not supply some moisture to the air. The lower air is never 
wholly free from moisture, the amount present depending chiefly 
upon the temperature of the air. Evaporation takes place at all 
temperatures. The moisture of the air is for the most part in its 
invisible form, that of water vapor; but it becomes visible when, 
by cooling, the vapor changes to the liquid or solid state as clouds. 
Real steam issuing from a boiling kettle is invisible. So-called 
steam, the visible cloud seen a few inches away from the spout, 
is composed of minute particles of water, and not water vapor. 

Humidity of the Air. — The air at all temperatures has a certain 
capacity for water vapor. Other conditions remaining the same, 
the higher the temperature the greater the capacity. To satisfy 
this " thirst " of the air evaporation goes on. If the temperature 
is low, or the capacity of the air is nearly satisfied, evaporation is 
slow; and when the capacity is fully satisfied, evaporation ceases. 
The air is then said to be saturated. 

The condition of the air as regards the amount of water vapor 
present is called its humidity. The actual amount of water va- 
por in a given volume of air, as the number of grains of water 
vapor in a cubic foot of air, is its absolute humidity.' The amount 
of water vapor present, divided by the amount the air is capable of 
holding at the time, is its relative humidity. Relative humidity is 
expressed in per cents. 



Ii8 PHYSIOGRAPHY 

If the air at a given temperature has six grains of water to the 
cubic foot of air present, and its capacity is ten grains to the cubic 
foot, its absolute humidity is " six grains per cubic foot," and its 
relative humidity is 60%. When the air is saturated its rela- 
tive humidity is 100%. The temperature of saturation is called 
the dew point. 

Condensation. — As evaporation is promoted by increase of tem- 
perature, so it is retarded by any lowering of temperature; and 
when the air is saturated evaporation stops, or if it continues con- 
densation goes on at an equal rate. 

Saturation may result from continued evaporation without 
change of temperature, or from a lowering of the temperature; 
and when the air is saturated, any lowering of the temperature 
results in a change from the vapor to the liquid or solid state, 
depending upon the temperature at which the change takes place. 
This change is condensation. It is condensation, not of the air, but 
of the water vapor of the air. 

The cooling which causes condensation may result from: 

1. Loss of heat by radiation, chiefly to the land or water from 
the lower air, when fogs are apt to result; 

2. Contact with cold surfaces, when dew or frost may form; 

3. Horizontal mixing of cold and warm currents, when clouds or 
precipitation may occur; 

4. Mechanical cooling by expansion, incident to ascending cur- 
rents, producing clouds or yielding precipitation. 

The last is probably the most effective cause of condensation. 
Air in rising expands and is mechanically cooled, at the rate of 
i.8° F. for every 300 feet of ascent. This rate is the same whether 
the air rises by convection, or is forced to ascend by reason of 
winds blowing against a rising land surface. 

When air descends, as at the center of a high, or on the leeward 
slope of mountains, it warms up at the same rate. 

The doldrum belt, the regions of lows, and the windward slopes 
of mountains are, therefore, apt to be rainy, while the high pressure 
calm belts, the regions of highs, and the leeward slopes of mountains 
have prevailingly clear skies. 



MOISTURE OF THE AIR 1 19 

If saturation is reached at a temperature lower than freezing, 
with further cooling the water vapor changes at once to the solid 
state, without passing through the liquid state; but if saturation 
occurs above the freezing point, the condensed product is liquid. 
As water vapor may change immediately to the solid state, so ice 
may evaporate without melting. 

It is a familiar fact that snow gradually disappears from the 
land during days when the temperature does not rise to the freez- 
ing point; and a wet garment hung in the air may freeze dry. 
Roads in winter become dusty though continuously frozen. 

Effects Upon Temperature. — In order to change ice or water to 
vapor, heat is absorbed, or becomes latent. This heat is used in 
the mechanical task of driving the molecules of ice or water apart, 
and is lost, as far as affecting temperature is concerned. The 
vapor formed is of the same temperature as the substance from 
which it came. The heat necessary for evaporation is obtained 
from surrounding objects, chiefly from the lower air, so that 
evaporation has the effect of cooling the air. Evaporation is always 
a cooling process with respect to surrounding objects. 

The custom of fanning to cool one's self is an illustration of 
cooling by evaporation, the constantly changing air in contact 
with the moist skin taking up the moisture more rapidly. 

A thermometer suspended in front of a rapidly rotating fan registers 
a slightly increased temperature, there being no evaporation from its 
surface, and the air being slightly compressed against the thermometer. 
If, however, the thermometer bulb be wrapped in a thin piece of moist 
muslin, the draft from the fan causes a marked lowering of the ther- 
mometer. 

Gasolene, bay rum, or any easily vaporized liquid, when rubbed upon 
the hands cools them; and the high fever temperatures may be cooled 
by alcohol baths. The flesh may be frozen by the application of ether. 

The principle of cooling by evaporation is used in the construc- 
tion of the psychrometer, an instrument for the determination of 
the humidity of the air. It consists simply of a wet and a dry 
bulb thermometer. By keeping the bulb of one thermometer con- 
stantly moist, evaporation from the moist surface cools the mer- 



120 



PHYSIOGRAPHY 



cury in the enclosed bulb. The wet bulb thermometer, therefore, 
commonly shows a lower temperature than the dry bulb. The 
more rapid the evaporation the greater the difference in the read- 
ings of the two thermometers, and the drier the air. Little differ- 
ence in the readings indicates slow evaporation and a humid air. 
When evaporation ceases, as when the 
air is saturated, the two thermometers 
show the same readings. 

When the reverse process takes place, 
and vapor is changed to liquid or solid, 
the latent heat of evaporation is released; 
and the heat thus liberated becomes 
available for affecting temperature. 
Surrounding objects are then warmed. 
Condensation of water vapor is, there- 
fore, a warming process. 

A burn from steam at 212° F. is much 
more severe than a burn from water at 
the same temperature, since a great 
amount of heat is liberated in reducing 
steam to a liquid without changing its 
temperature. 

Distribution of Water Vapor. — Since 
most evaporation takes place at the bot- 
tom of the air, it follows that the lower 
air is of higher absolute humidity than 
the air at greater altitudes. Also the air over the oceans and 
other water surfaces is more humid than the air over the land. 
But by diffusion and by vertical currents the water vapor is 
distributed generally through the lower air to a height of about 
seven miles. 

Although there is more water vapor in the lower air, the relative 
humidity of the lower air is not usually so high as that at greater 
altitudes, because of the lowering of temperature with increase of 
altitude. Clouds and rain are usually the result of condensation in 
the upper air, since the air cools in rising, due to expansion. This 




FlG. 57. — PSYCHROMETER 



MOISTURE OF THE AIR 1 21 

is perhaps the most important, though not the only cause of con- 
densation of water vapor. 

The relative humidity of the air varies, not only with change 
of place or of altitude, but it also varies from hour to hour 
at any place. It is highest at the coolest hour of the day, 
usually in the early morning, and decreases as the day ad- 
vances and the air warms up. It is lowest at the warmest hour 
of the day, from one to three o'clock P. m., after which hour it 
slowly rises. 

Dew and Frost. — When air at temperatures / above the dew- 
point is cooled to saturation by contact with objects below the 
dew-point, condensation upon the cooling object occurs. If satu- 
ration occurs above freezing, the condensation is liquid, and we call 
it dew. If saturation occurs below freezing, the vapor changes at 
once to ice, without visible liquefaction, and this is called frost. 
Dew and frost form; they do not fall. Frost is not frozen dew, frozen 
dew being transparent pellets of ice. 

These forms of condensation are most common on or near the 
surface of the ground. At a height of five feet there may be no 
frost or dew formed on plants when the ground and objects near 
it, as the grass, are white with frost or wet with dew. 

Anything that checks the cooling of the ground and lower air 
tends to prevent the formation of dew and frost. The ground 
beneath trees and shrubs is often protected from dew and frost 
when these form freely upon the unprotected ground. A cloudy 
sky, by checking radiation, prevents excessive cooling and hinders 
the formation of dew or frost. Likewise wind, by constantly 
changing the layer of air next to cold surfaces, hinders cooling and 
condensation. It is rare to have frost or dew on cloudy or windy 
nights. 

It is a commonly recognized fact among dwellers in the country 
that clearing skies, and the " laying " of the wind toward night- 
fall, may bring frost at those seasons when early planted or late 
maturing crops would be injured by it. 

Many orchardists protect their trees from frost by building 
smudges among the trees. The smoke cloud thus produced, 



122 



PHYSIOGRAPHY 



hanging above the orchard, checks radiation as a blanket would, 
and thus often prevents frost. 

When condensation begins, the liberation of latent heat tends 
to check further cooling; so if saturation occurs much above 
freezing, frost is unlikely because freezing temperatures are not 
apt to be reached. 

Housewives often protect their flowers from frost, on cold 
nights, by exposing shallow pans of water in the warm room with 




Fig. 58. — Cirrus Cloud 

the plants, and thus greatly increasing the humidity. When the 
room cools saturation will be reached at a temperature well above 
freezing; and the liberated latent heat, as condensation goes on, 
may suffice to keep the temperature above freezing. In a few 
cases this principle has been applied on a large scale for the pro- 
tection of orchards. 

Clouds. — When the vapor in the air is condensed above the sur- 
face of the ground, into particles of water or ice so small that they 
remain suspended in the air, the product of condensation is called 
cloud. 



MOISTURE OF THE AIR 



123 



If the cloud is so low that it reaches the land or water it is called 

fog- 
Clouds are classified chiefly by their form. The thin, feathery, 
white clouds, seen high in the air, and frequently on fair days, are 
cirrus clouds. These filmy clouds are, for the most part, com- 
posed of ice spicules, and are the highest clouds. Their average 
summer altitude is about 6 miles; and they generally move east- 




Fig. 59. — Cumulus Cloud 

ward at the rate of about 60 miles an hour. In winter their average 
height is about 5 miles, and their eastward motion is about 100 
miles an hour. 

Cirrus clouds are often precursors of storms, being the high- 
level overflow in front of the cyclone center. 

Cumulus clouds are the massive piles of cloud, with an even 
base, and resembling piled- up fleeces of wool, or. volumes of con- 
densed steam and smoke from a locomotive. They are the result 
of rising currents of air, usually of convectional origin, and are, 
therefore, storm clouds. They are sometimes called thunder-heads, 



124 PHYSIOGRAPHY 

and are land clouds rather than sea clouds, day rather than night 
clouds, and are more commonly in motion than at rest. 

The average summer height of cumulus clouds exceeds a mile, 
whereas their winter height is somewhat less; and their summer 
and winter velocities are about 6 and 9 miles an hour respectively. 
Their even bases are usually from one-fourth to one-half mile high. 

Nimbus is the name given to any cloud from which rain or snow 
falls or may be expected to fall. These clouds are of very variable 
height, being, on the whole, higher in summer than in winter. 
Their height decreases toward the poles. 

Nimbus clouds are of an even grayish tint, sometimes completely 
overcasting the skies for hours and even for days continuously. 

Stratus is any low-lying cloud spread out in parallel sheets or 
bands. It is a night rather than a day cloud, and more common 
over the sea and in valleys than over the higher lands. It includes 
fogs. The bands of clouds seen at higher levels may be designated 
by such compound names as cirro-stratus and cumulo-stratus. 

Fog is the cloudy condensation near, or resting on, the land or 
sea. It usually results from warm, humid air passing over cold 
surfaces. In winter, winds blowing from the sea upon land are 
apt to produce fogs. An east wind upon eastern coasts, and a 
west wind upon western coasts are the chief sources of fogs. 

Fogs are more frequent in valleys than on the slopes or tops 
of hills and mountains. The cooler, heavier air accumulates in 
the valley, where there is likewise more moisture; and these two 
conditions combine to produce the fogs in valleys and lowlands. 

The fogs of Newfoundland are known to all navigators of the 
North Atlantic. The warm and cold ocean currents or drifts 
which meet there are accountable for the fogs. When the winds 
are from an easterly quarter their temperatures are lowered and 
their moisture condensed as they pass over the cold Labrador cur- 
rent to the west. Another cause of much of the Newfoundland 
fog is the ice brought down from the Arctic regions by the Labra- 
dor current. These icebergs are apt to be centers of dense fogs, 
especially in summer. 

The fogginess of great cities, as London, and particularly of 



MOISTURE OF THE AIR 



125 



manufacturing cities, is greatly increased by reason of the particles 
of soot issuing from chimneys. Each soot particle is a center of 
cooling and condensation. 

Rain and Snow. — When condensation takes place in the air, and 
the condensed particles become heavy enough to sink through the 
air, the process is called precipitation. If saturation occurs above 




Fig. 60. — Valley Fog 

freezing, the product is rain; if below freezing, snow. As in the 
production of frost, so in snow, the water vapor passes at once 
from the vapor condition to the solid. Rain and snow bear the 
same relation to each other as do dew and frost. Snow is not 
frozen rain. 

Although clouds almost invariably precede rain and snow, precipitation 
without clouds may occur. Such cloudless rain, usually in very fine 
drops, and more resembling mist, is called serein. 

Snowflakes are crystallized water vapor, and are built up on patterns 
resembling beautiful six-rayed stars. The size of the flake as well as 
the exact pattern seems to depend upon the temperature at which the 
flake is formed. 

In all latitudes an altitude may be reached at and above which 



126 PHYSIOGRAPHY 

the precipitation, even in summer, is chiefly in the form of snow. 
At a slightly lower level more snow falls than melts during the 
year, with the result that there is perennial snow. The lower 
limit of perennial snow is known as the snow line. It is about 
16,000 feet above the sea at the equator, and descends gradually 
toward the poles, reaching sea level within the polar circles. 

Sleet and Hail. — If raindrops pass through increasingly colder 
air in their fall, and are frozen into small pellets of ice, the product 
is sleet. As this arrangement of temperature is most apt to occur 
in winter, when the land is colder than the lower air above it, sleet 
is a winter phenomenon. Sleet is frozen rain. 

In summer, especially on hot afternoons, and near the center of 
a cyclonic storm, large pellets of ice called hail often fall. Upon 
examination hailstones prove to be made of concentric layers of 
ice. This structure, together with their often great size, suggests 
that they are frozen raindrops enlarged by successive condensa- 
tions and freezings upon their surfaces. Their often spongy tex- 
ture indicates that snow may also enter into their composition. 
Hail is chiefly a summer phenomenon. 

Hail storms are sometimes very destructive. Their paths are 
usually only a few miles in width, and fortunately not of great 
length; for often growing crops, orchards, and even forests are 
destroyed. Leaves, bark, and branches are stripped from trees; 
young animals are killed, and windows and roofs broken by the 
hailstones. 

It has been suggested that hail is rain frozen by being carried in strong 
ascending currents to higher, freezing altitudes. The occurrence of 
hail near the storm center, where the convectional ascent is strongest, 
supports this claim. 

It is also suggested that the meeting of cold and warm masses of air 
may result in horizontal stratification, warm and freezing layers alternat- 
ing; and that raindrops formed near the top of such a stratified cloud 
are frozen and enlarged in passing through it. The addition of snow- 
flakes, formed in the colder layers of the cloud, would explain both their 
sponginess and size. Hailstones more than nine inches around have 
been reported. 



MOISTURE OF THE AIR 



127- 




128 PHYSIOGRAPHY 

Sheet-Ice. — Sometimes, in winter, rain falls, but immediately 
upon touching the ground or trees it is changed into ice. This 
occurs when the lower air is just above the freezing point, while 
the ground and all solid objects near it, being better radiators, 
have cooled to a temperature below freezing. This is called 
sheet-ice. 

During such ice storms ice encases the twigs and boughs of 
trees and shrubs; and sometimes the weight of ice is sufficient 
to break the branches. Such storms are especially destructive if 
strong winds occur before the ice disappears. Telephone and 
telegraph lines are often broken down. 

QUESTIONS 

i. Precipitation increases for a time, then decreases with increase of 
altitude upon the slopes of most mountains; give reasons. 

2. Why is it that if we determine the humidity of the air when it is 
raining we rarely find the air saturated? 

3. The relative humidity decreases, usually, as the day advances, 
until 1 or 2 o'clock; explain. Does the absolute humidity decrease in the 
same way? 

4. Why is the relative humidity of the air 10 feet above the ground 
usually higher at night and lower during the day than that of the air 100 
feet above the ground? 

5. Why is the absolute humidity greater over, and near, the sea than 
inland? 

6. Why does one become so much more quickly chilled in a wet gar- 
ment than in a dry one? 

7. Why, in winter, does frost form on the window panes of an occupied 
room, but does not, if the chamber is unoccupied? Why will thorough 
ventilation of the occupied room prevent such formation? 

8. Why do fogs form over lakes before they form over the surrounding 
lands? How are such fogs dissipated as the day advances? 

9. Rain may often be seen falling from clouds, yet not reaching the 
ground; what becomes of it? 

10. Why is there no distinctly wet or dry coast under the equator? 



CHAPTER XII 
LIGHT AND ELECTRICITY OF THE AIR 

Introduction. — We see the sun at rising as a golden sphere, at 
mid-day a globe of dazzling white, and at setting, if the air is 
dusty, it may disappear below the horizon as a ball of fiery red. 
As we ascend the mountain slope the noon-day sun takes on a 
bluish tinge; and we are told that balloonists, in their highest 
ascents, see a distinctly blue sun. 

The ever-changing color of the sun, as it mounts toward the 
zenith, or shines through clear or cloudy air, is the result of the 
irregular reflection of the light. White light is composed of many 
colors, each having its distinct length of ether wave, and because 
of these different wave lengths white light may be separated into 
its component colors. The shortest waves are blue, and like rip- 
ples upon water are easily turned aside by obstacles in their path; 
the longest visible rays are red, and these, like the great waves of 
the sea, pass by obstacles which send back shorter waves. Hence 
it follows that an object may appear one color by reflected light, 
and quite a different color by transmitted light. 

A glass of soapy water, viewed from above, looks bluish white, 
but when the sun is seen through the water its color is red or red- 
dish yellow. The short blue waves are turned back, whereas the 
longer reds pass by the minute solid particles in the water. 

Color of the Sky. — The dust and cloud particles in the air reflect 
the shorter waves, and transmit the longer; and the freer the air 
from dust and cloud, the more completely do light waves of all 
lengths pass through it. To an observer looking toward the sun 
the irregularly reflected or diffused light is lost, and only the longer 
waves reach the eye. If the shortest blues alone are scattered, as 
when the air is moderately clear, the combination of the remaining 



I 3 o PHYSIOGRAPHY 

colors gives the sun a yellowish tinge; but if the light passes through 
a very dusty air, or through a considerable thickness of cloud, the 
sun takes on a distinctly reddish tinge. 

If the eye is turned away from the sun the diffused light is 
received; and as the blue rays are in excess in diffused light, the 
sky appears blue. The less the admixture of other colors with the 
blue the deeper the blue; and for this reason the sky, as seen from 
a balloon or mountain top, above the dust-laden stratum of air, 
appears intensely blue. 

If the air is very dusty many other colors are diffused with the 
blue, and the sky assumes a whitish glare. The sky is bluer at 
sea, and after a rain, because the air is freer from dust. It is 
believed that if we were to rise above all the dust and cloud par- 
ticles the sky would have the blackness of night; and the stars 
would shine as brilliantly by day as by night. 

Refraction in the Air. — As a ray of light is bent in passing 
obliquely from one medium to another of different density, so all 
the sun's rays are bent in passing through the atmosphere, except- 
ing only the vertical ray. This bending, called refraction, increases 
with increasing obliquity of the ray, being greatest when the sun 
is on the horizon, and least when on the meridian of a place. At 
sunrise and sunset it is sufficient to displace the sun the width of 
its disc; and as its effect is always to increase the altitude of the 
sun, the entire disc of the sun appears to be above the horizon 
when in reality below it. 

The effect of refraction is to increase the length of the day in 
all latitudes. While this increase amounts to only a few minutes 
at the equator, at the poles it amounts to about four days. At 
the time of the equinoxes the sun is wholly above the horizon at 
both poles. The day at each pole is thus lengthened by about 
four days, the polar night being shortened by this amount. An- 
other effect of refraction is to give the sun, at rising, an elliptical 
appearance, flattened vertically. 

Looming. — Normally the air is densest at the bottom, becoming rarer 
with increase of altitude. Sometimes the lower air, for a thickness of a 
few feet, is abnormally cooled and denser than the air ten or twenty feet 



LIGHT AND ELECTRICITY OF THE AIR 



131 



higher. This is apt to occur in the early morning in summer, particularly 
over the land, owing to the rapid radiation of the land at night, and the 
cooling of the air near it. 

Rays of light coming from an object that rises above the cooled stratum 
of air to an observer within it are bent downward; and the object is seen 
as occupying a position higher than is real. Sometimes objects appear 
suspended in air, in their normal upright position; but more often they 
are merely elongated upward. This is looming. 




Fig. 62. — Looming 



In Fig. 62 above the surface A B the air to the height of C D is 
abnormally cooled; coldest at the bottom, and becoming warmer and 
rarer above C D. From the object X Z which rises above C D rays 
come to the observer at E, within the denser layer of air. These rays are 
bent downward upon entering the denser layer, making the part of the 
object above C D appear to occupy a higher position. If the object is 
distant, only the upper part is seen; but as the object is approached it 
descends, until finally seen as an elongation of X Z. 

Looming is an early morning or winter phenomenon, and ships at sea 
are often discovered while yet below the observer's horizon. 

Total Reflection.— When a ray of light passes obliquely from a denser 
to a rarer medium, the ray is bent away from a perpendicular to the sur- 
face of the media at the point of passage. There is an angle of obliquity, 
varying with the densities, beyond which the ray will not pass from the 
denser to the rarer medium, but will be totally reflected from this surface 
back into the denser medium. This angle is called the critical angle, 
and this phenomenon is known as total reflection. 

Mirage. — The interesting phenomenon of mirage depends upon the 
principle of total reflection in the air. Often in deserts or upon arid 
plains, during a hot summer day, the air resting upon the land becomes 
greatly heated, and rarer than the air above. If the day is calm, con- 
siderable difference in density may be developed before convectional 
motion sets up. When this condition of things exists, travelers often 
see distant objects reflected as from a water surface. In reality the 
reflection is from the upper surface of the rare stratum of air. 

In Fig. 63, C D is the upper surface of a thin, rare layer of air overly- 



132 



PHYSIOGRAPHY 



ing A B, and distinctly rarer than the air above. A distant object, as 
X Z, rises above C D. The angle at which the rays from the object 
strike the surface C D is greater than the critical angle, and the rays 
are totally reflected. The object is then seen as in the position X Z', as 
reflected from a water surface. The object and its image are both seen. 
This species of mirage is peculiarly a land phenomenon, and is seen 
in hot weather, and oftenest in low latitudes. 




Fig. 63. — Hot Weather, Midday Mirage 



Another species of mirage is seen over the sea as well as upon land, 
occurring in winter, and most frequently in high latitudes. 

In Fig. 64 the air up to the height of C D is distinctly colder and denser 
than the air above C D. This dense stratum is of such thickness that a 
vessel or city may be wholly immersed in it. Rays from the distant 
object strike the under surface of the rare overlying layer at such an 
angle that they are totally reflected to the observer at E. He thus sees 
the reflection of the object in the inverted position shown, above its 
real position. 

The city of Baton Rouge, Louisiana, was thus seen by observers thirty 
miles away; and travelers in Arctic waters frequently report seeing vessels 
thus inverted and suspended in air. This usually occurs near the land, 
and is due to an outrush of cold air from the snow-covered land over the 
adjacent sea. Being heavier, it underruns the less dense air. 

Halos. — Sometimes, in front of a cyclone, when the cirrus clouds that 
outrun and foretell the coming storm stretch across the sky, light or 
colored rings are seen encircling the sun. These rings, of varying diam- 
eter, are called halos. They are believed to result from refraction and 
reflection of light, by the ice crystals that make up the cirrus cloud; or 
diffraction by these crystals, or by the dust and cloud-mist at lower 
levels. If the cloud sheet is thick the rings are nearer the sun. Similar 
rings and luminous areas are observed about the moon. 

Sometimes a more complex system of intersecting rings occur, which, 
though too dim to be seen throughout their whole extent, are visible at 
their points of intersection or tangency, where they appear as bright or 



LIGHT AND ELECTRICITY OF THE AIR 



133 



colored spots. These spots are systematically arranged with reference 
to the sun, and are known as mock suns or sun dogs. 

Halos and mock suns are commonly thought to portend coming 
storms. They are particularly bright and their occurrence frequent in 
high latitudes. 

1 .,?¥■„ ' 



\/ / 
JfZ 



i'i 1 1 ti l It '1 1 1 hi iVti'htII 1 
! 1 1 1 III II 1 II Ml 




Fig. 64.— Cold Weather, Early Morning Mirage 



The illuminated areas encircling street lamps on foggy or misty nights 
are analogous to the halo, as are also the brilliant borders of dense cloud 
masses lying between the observer and the sun. 

The Rainbow. — If an observer stands with his back to the sun 
while rain is falling in front of him, there often appears the arc of a 
circle, made up of parallel bands of colored light, called the rainbow. 
It results from refraction and reflection of light by the raindrops. 

By refraction and dispersion the white light is decomposed into 
its constituent colors. Red, being the least refracted, lies upon 
the outer side of the bow, while blue, being most refracted, occu- 
pies the inner side. The angle formed by drawing lines from the 
eye to the ends of the bow's diameter is usually about 82 degrees, 
and the center of the bow lies upon the straight line passing 
through the sun's center and the observer's eye. On this account 
the bow is usually less than a half circle, unless seen from some 
high point, as a mountain peak. 

Though commonly a daytime phenomenon, rainbows are some- 
times produced by moonlight. 



i 3 4 PHYSIOGRAPHY 

Colored bows, and even complete circles, analogous to rainbows may 
be observed in the spray of fountains and waterfalls. 

If favorably situated, with a sheet of calm water at his back, one may 
see two distinct and intersecting bows, the shorter produced by the direct 
rays from the sun, the longer by rays reflected from the water surface. 

A less distinct secondary bow, outside the more brilliant one, and with 
the order of the colors reversed, is sometimes seen. This bow is usually 
more than ioo degrees in diameter. 

The primary bow is thought to result from rays of light entering near 
the side of the drop, being refracted upon entering, totally reflected from 
the back surface of the drop, and again refracted upon leaving the drop. 
This disperses the colors, so that we receive but one color from any drop. 
The red is given by drops farther away from the line passing through the 
sun and the observer's eye than the drops that produce the blue; hence 
the red is on the outside of the bow. Since all drops which send a given 
color to the eye lie at the same angular distance from this line, it follows 
that the bands of color and the entire rainbow are arcs of circles, whose 
center lies upon this line. 

The secondary bow is thought to be produced by raindrops which 
receive the rays in such a way that they are twice reflected within the drop 
before leaving it. This double reflection accounts at the same time for 
the dimness of the bow, its greater diameter, and the reversal of the 
order of the colors from that of the primary bow. 

Lightning. — Every year records a considerable loss of life and 
property in the United States from lightning. Men and animals 
are killed, trees and houses shattered and often set on fire, and 
hay and grain stacks burned. Lightning storms, commonly called 
"thunderstorms," are usually associated with an overheated con- 
dition of the air, locally, and are most common in the afternoon 
of hot summer days. They occur not infrequently at night, and 
may even occur in winter; but their cause seems always to be 
found in a rapid convectional overturning of the lower air. 

The air is always electrified, but it is only when clouds are form- 
ing rapidly, as in the ascending currents near some storm center, 
that discharge takes place. This discharge, known as the lightning 
flash, may be between clouds, or it may be from cloud to earth. 
When downward those objects which rise highest, as buildings 
and trees, are most in danger of being struck by lightning, these 
being better conductors than air. 



LIGHT AND ELECTRICITY OF THE AIR 135 




Fig. 65. — Photograph of Lightning Flash 



In those sections of the country where wire fences are exten- 
sively used great numbers of stock are annually killed by light- 
ning, from collecting near these fences during thunderstorms. 



136 PHYSIOGRAPHY 

The wire serves as a conductor of the lightning horizontally, often 
for considerable distances, being finally led to the ground by the 
fence posts, which are thereby shattered. Telegraph and telephone 
poles are shattered in a similar fashion. The belief that " lightning 
does not strike twice in the same place " is a dangerous error. The 
fact that lightning strikes in any given place argues the existence 
there of favorable conditions. 

Protection from Lightning. — The fear of lightning, and the de- 
sire for immunity from it, have led to the adoption of many pro- 
tective measures. Perhaps the most common artificial protection 
is the lightning rod. 

As all solids are better conductors of electricity than the air, buildings 
and trees are more often struck by lightning than the open surface of 
the ground near them. The greater the number of buildings or trees 
among which the discharge may be divided, the less the individual liabil- 
ity. On this account a house in the city has greater immunity from 
lightning than the isolated farm house ; and any tree in the forest is safer 
than the "lone tree" upon the prairie. 

Where country houses are surrounded by shade trees these offer per- 
haps the best protection from lightning. Each individual tree becomes 
a means for silently discharging the passing clouds of their electricity, 
thus preventing heavy and destructive discharges. 

The lightning rod, as a protection against lightning, has been in use 
almost ever since the discovery of the nature of lightning. Its use is 
based on the theory that metal, being a better conductor of electricity 
than the building, by affording an easier route, will prevent the discharge 
from passing to the building itself. It is also thought that the numerous 
points which rise above the building may quietly drain away the electric 
charge from the clouds, and thus prevent a destructive discharge. 

The lightning rod consists, usually, of a metal ribbon or flattened tube, 
commonly of copper or galvanized iron, laid over the roof of the building, 
with frequent branches rising from six to ten feet in air. These branches 
end in one or more sharp points; and the rod should extend sufficiently 
deep into the ground to reach moist earth. If it ends in a cistern of 
water so much the better. The greater the number of branches rising 
above the building the better, as the discharge is thereby more divided. 

Perhaps the greatest security from lightning is obtained by encasing 
the structure in a network of wire. 

Kinds of Lightning. — Zigzag lightning is a very common form. The 
course of the flash is probably a sinuous one, and only appears angular 



LIGHT AND ELECTRICITY OF THE AIR 



137 



when seen along the direction of its path. Ramified or branching light- 
ning may begin and end in a multitude of branches, uniting in a trunk 
flash between. This kind of flash takes place between clouds. Heat 
lightning, and sheet lightning, probably the same, are believed to be the 
illumination of cloud masses so far away that the accompanying thunder 
is not heard. St. Elmo's Fire is the name given to the discharge of 




ree Destroyed by Lightning 



atmospheric electricity as a brush of bluish flame, often observed at the 
ends of masts and spars of ships, at tree-tops, or house-tops, or any 
pointed object. A crackling sound accompanies it, similar to that of the 
artificially produced electric spark. 

Thunder. — The production of thunder may be likened to the pro- 
duction of the noise accompanying the explosion of gunpowder. Along 
the path of the lightning flash the air is intensely heated and pushed 
away. The collision of the returning air particles, causing but a crack- 
ling, of high pitch, in the case of a few short sparks, becomes a crash of 
lower pitch in the case of a lightning flash, which is but a great number 
of longer sparks. The quick succession of crashes following along the 
path of the flash unite to produce the roar; and the roar, when echoed 
and re-echoed from cloud masses, gives the rolling so often observed to 
follow brilliant flashes of lightning during thunderstorms. 

Relation of Lightning to Rain. — The condensation of the moisture 
in the air forms cloud particles, and the clumping of these particles, by 



138 PHYSIOGRAPHY 

reason of their mutual attraction, forms raindrops. With increase in 
size of the raindrops the electric charge of the drop is increased, and 
lightning discharge is made possible. 

It would seem that lightning, for the most part, precedes the rain. 
When the raindrops begin to fall each carries down with it a minute 
charge, and in this way the cloud mass is discharged. Further produc- 
tion of lightning is then impossible. The heaviest fall of rain in a thunder 
shower often follows closely the most brilliant lightning and heaviest 
thunder. So while lightning mainly precedes the rain, it seems probable 
that they are mutually cause and effect. 

The Aurora. — This is the beautiful electrical display, common in 
high latitudes, though often seen in northern United States. In the 
northern hemisphere it is called A uroraBorealis, and in the southern, 
Aurora Australis. It is believed to be due to the discharge of elec- 
tricity into the rare upper air, and seems to bear some relation to 
sun spots. The Aurora lessens the gloom of the long polar night. 

As seen in the United States the Aurora, also called Northern Lights, 
usually consists of a more or less distinct arch of light, extending east 
and west, snd crossed at right angles by streamers of colored light which 
radiate from a point in the northern horizon. The arch is highest above 
the magnetic meridian; and the streamers of red, yellow and green light 
change their position and length, and are called "the merry dancers." 

Brilliant aurora displays are often accompanied by severe electrical 
and magnetic disturbances throughout the country. The telegraph and 
telephone services are often interrupted for hours; and the magnetic 
compass sometimes becomes so variable as to be useless. 

QUESTIONS 

1. Why do we not ordinarily observe the phenomenon of "looming" 
at midday? 

2. Would the phenomenon of " looming " be dispelled by ascending 
into the air? 

3. In the mirage resulting from the lower air being cooler than that 
above, need the object, the image of which is seen, be visible? 

4. It is a common notion that the greater the number of stars visible 
within the halo rings about the moon, the greater the number of days 
before the storm, which these rings presage, will arrive; what scientific 
grounds exist for this belief? 

5. How did Franklin discover the identity of lightning with the arti- 
ficial electric spark? 

6. Why should we avoid the shelter of tall trees in a thunderstorm ? 
Why do we disconnect our telephones during thunderstorms ? 



CHAPTER XIII 
WEATHER AND CLIMATE 

Weather and Climate Defined. — Weather is the condition of the 
air at a given time and place with reference to temperature, mois- 
ture, state of the sky, and winds. These conditions, called weather 
elements, are constantly changing; and as a consequence, for most 
places in temperate latitudes, the weather is proverbially fickle. 

As the day advances, after sunrise, the temperature normally 
rises, reaching its maximum between one and three o'clock 
p. M., after which it falls till near sunrise the following day. With 
these changes in temperature come changes in the relative hu- 
midity, and usually changes in wind direction and strength. 

Climate is the average condition of the air with reference to tem- 
perature, moisture, state of the sky, and winds; or it is average 
weather. While in weather we consider current temperatures, in 
climate maximum and minimum temperatures are of more im- 
portance. 

We use the term weather in referring to atmospheric conditions 
at any given instant; also for such short periods as a day, a week, 
or a month. We even speak of " summer " or " winter " weather; 
but when we apply the term to these longer periods we refer rather 
to the average conditions during these periods. 

Pressure, though exercising a controlling influence upon weather 
elements, is not itself commonly counted among them. 

Weather Changes. — Although the variability of the weather is 
proverbial, yet there are important controls, which, by reason of 
their orderly sequence, give a certain degree of system to the suc- 
cession of weather changes. The most important of these are: 

i. The alternation of day and night, due to rotation; 

2. The annual succession of winter and summer, due to revolu- 
tion; 



1/j.O 



PHYSIOGRAPHY 



3. The more or less systematic passage of lows and highs. 

The first two are fairly regular in period and value at any place, 
though widely differing for different places; whereas the third 
varies in both period and intensity. 

The daily and annual changes of the weather are more pro- 
nounced near sea level than at higher altitudes, in the interior 
of the continent than near the coast, and in high than in low 
latitudes. As a rule the temperature rises and the absolute hu- 
midity increases during the day and in summer, both being lower 
at night and in winter. 

Convectional cyclones are more frequent over the land than 
over water, and more vigorous in summer than in winter and in 
the daytime than at night; whereas non-convectional cyclones are 
most frequent and intense in winter. Both classes of cyclone are 
probably more highly developed, and likewise of longer duration, 
over the sea than over the land. 

In the United States during March and April, when the land 
is warming up most rapidly, it is a common occurrence to have 
days of blustery winds succeeded by nights of calm. This is due 
to the rapid warming and convectional overturning of the lower 
air during the day. 

Weather in the Tropics. — Night has been called the " winter of 
the tropics." This is because the variation in weather conditions 
from day to night is greater than from winter to summer. 

In the doldrum belt, the days are uniformly warm, owing to the 
nearly vertical rays of the sun; and the rapid convectional ascent 
of the air in the morning is usually followed later in the afternoon 
by torrential downpours of rain, followed in turn by cloudless 
nights. The nearly equal day and night, combined with the low 
percentage of cloudiness, accounts for the great daily range of 
temperature. Cyclonic interruptions are of secondary importance. 

In the trade-wind belts, over the sea, there is a constancy of 
weather conditions not found elsewhere. The extreme range of 
temperature scarcely exceeds ten degrees; and the wind blows 
continuously from the same direction, and with about the same 
strength day and night. On land the range of temperature in- 



WEATHER AND CLIMATE 141 

creases, and over both land and sea there is little rainfall, except 
where the winds are compelled to rise over the ascending land. 
This is due to the fact that the trades grow warmer as they ad- 
vance. 

Regions on the borders of the trades have monsoon changes of 
weather. If next the doldrums, there is the alternation of the 
light winds and abundant rains of the doldrums, and the constant 
winds and light rains of the trades. If next the high pressure 
calms, then the characteristic conditions of the trades alternate 
with the prevailing calms of the horse latitudes. 

Weather Outside the Tropics. — In the zone of prevailing wester- 
lies weather changes are irregular, and mainly of cyclonic control, 
with marked differences in the two hemispheres. In the southern 
hemisphere, where there is little land to interrupt them, the 
westerlies attain a constancy approaching that of the trades; and 
so high a velocity that they are called the " Roaring Forties." 
In winter the cyclones are more frequent and succeed each other 
with almost periodic regularity. 

In the northern hemisphere, where the land is massed, there is a 
strong contrast between the weather of the land and water areas 
of the prevailing westerlies, the land areas having much greater 
extremes of w r eather conditions, both daily and seasonal. 

In the frigid regions, although temperature changes are deter- 
mined chiefly by the appearance and disappearance of the sun, 
the other weather elements are controlled mainly by the passage 
of cyclones. 

Weather Prediction. — After a thorough understanding of the 
relative values of the factors determining weather in any region 
it is possible to predict, with a high degree of accuracy, the changes 
of weather likely to occur. The degree of accuracy attainable 
varies with the season and with geograpJhic position. Under the 
doldrums and trade winds, where the diurnal change is dominant, 
weather prediction may be made with an assurance almost amount- 
ing to a certainty that it will be fulfilled. Indeed, the weather 
changes there are so regular and certain that the weather is not a 
topic of conversation. 



142 PHYSIOGRAPHY 

In regions where the control of the weather is mainly cyclonic, 
it is not possible to predict with nearly so high a degree of accu- 
racy. Yet even here the relative values of the factors are so well 
known, and the systematic movement of cyclonic disturbances so 
well understood, that predictions may be made with the reason- 
able expectation that a large per cent of them will be fulfilled. 
These predictions for any station must take account of: (i) The 
systematic movement of cyclonic disturbances, their strength of 
development and place of origin, and direction and rate of move- 
ment; (2) The season; and (3) Local topography. 

Weather in the Cyclone. — To understand the weather condi- 
tions which prevail about lows and highs it is necessary to remem- 
ber the directions of the winds about these disturbances, and the 
effect upon the humidity of the air resulting from a change of 
temperature. 

In the United States cyclones, as we have seen, move eastward, 
and the winds blow in toward the cyclonic center, in counter- 
clockwise spirals. At any station the wind will not, as a rule, be 
blowing directly toward the center, but a little to the right of it. 
Therefore, in front of the cyclone the winds are blowing from a 
warmer to a cooler latitude, and their relative humidity is increased. 
As they approach the center of the low the air rises, and its humidity 
is further increased by cooling from expansion. This may be suffi- 
cient to bring the air to saturation. As a result of these conditions 
a rising temperature, with cloudiness or precipitation, generally 
characterizes the front of the low, and may be predicted as a well- 
developed cyclone approaches. 

In the rear of the cyclone the winds are moving from colder into 
warmer regions, and as a result the relative humidity of the winds 
is lowered. As they near the center of low and begin to rise, their 
temperature falls as a result of expansion; but the cooling must 
first counteract the warming due to their moving into warmer lati- 
tudes before their relative humidity reaches that possessed by the 
winds when they were inaugurated. 

As a result of this difference in conditions in front of and be- 
hind the cyclone the increase in humidity, due to ascent, may 



WEATHER AND CLIMATE 



143 



bring the air in front of the cyclone's center to the saturation 
point, and yet not saturate the less humid air in its rear. 

Consequently falling temperatures and clearing skies may be 
expected after the center of a well-developed cyclone passes. 

The direction of the shifting of the wind depends, as we have 
seen, upon the position of the path of the cyclone's center, whether 




Fig. 67. — An Ideal Low, Showing the Distribution of the Weather Elements About 
Its Center. (After Davis) 
D E F. ABC. and G H J are paths of observers through the storm area where the storm passes 
north, centrally over or south of the observer respectively. Note the succession of winds 
each will experience, also the probability of precipitation for each. Observe that the southerly 
winds in front, and the northerly winds behind the storm center, give the isotherms a general 
north-east -south west trend, and the observer will not experience a very severe change of tem- 
perature. If the storm passes centrally over him he will experience a lull in the wind near 
the center, after which the wind springs up from the opposite direction and increases in 
strength. 



north or south of the station. Ordinarily the strength of the wind 
increases as the cyclone approaches, and decreases' as the cyclone 
recedes. 



144 PHYSIOGRAPHY 

In winter the strong indraft of cold air in the rear of a cyclone, 
if accompanied by snow, is known as a blizzard. 

Weather in the Anti-Cyclone. — Since the movements of the air 
about a high are the reverse of its movements about a low, it fol- 
lows that the conditions as regards temperature and humidity 
which prevail about a high are likewise the reverse of those which 
prevail about a low. In front of a high the winds are northerly, 
and behind a high the winds are from some southerly quarter, 
while at the center of the high the air is sinking. Consequently 
fair and cooler weather is predicted, usually, as the high ap- 
proaches, and rising temperatures with possible cloudiness or even 
precipitation as the high recedes. 

Since the winds start from the center of the high, unlike the low, 
the winds weaken as the high approaches, and strengthen as it 
recedes. As with the low, the direction of the shifting of the 
wind is determined by the position of the station with reference 
to the path of the center of the high. 

In winter, if a high follows closely in the wake of a well-developed 
low, the fall in temperature may be abnormal. If it is as much as 
20 degrees F. in 24 hours, reaching a temperature of zero or lower, 
it is called a cold wave. In the southern part of the United States 
the term is applied to changes that are somewhat less than 20 
degrees, and that reach a somewhat higher minimum. 

Thunderstorms and Tornadoes. — In summer, after a day or so 
of excessive heat, the rapid convectional ascent of the air about a 
low may set up, locally, a more limited though more intense cy- 
clonic whirl. The rapid condensation of vapor in the rising and 
cooling air may give rise to, or be accompanied by, brilliant displays 
of lightning and heavy thunder. Such storms are known as thunder- 
storms. Torrential downpours of rain may follow quickly after 
the most brilliant discharges of lightning; but it is a notable fact 
that the lightning flashes become rapidly less frequent after the 
rain begins to fall. 

Thunderstorms are usually summer and day-time phenomena, 
though they sometimes occur in winter and at night. They are 
much more common in front of lows than behind them. In the 



WEATHER AND CLIMATE 145 

United States they occur most frequently in the southeastern 
quadrant of the low pressure area. 

If the local whirl thus developed is destructive in violence, it is 
called a tornado. The destructive path of a tornado is rarely a 
mile in width, and usually but a few miles in length; more com- 
monly it is but a few hundred yards in width. Within that nar- 
row path the violence of the wind is such that few structures above 
ground are strong enough to withstand it. In those States in the 
Middle West, where tornadoes are most frequent, underground 
structures called " cyclone-cellars " are built. These seem to offer 
the greatest security from danger. 

Tornadoes progress normally in a northeastern direction, at a 
rate of twenty or thirty miles an hour; whereas the spiraling 
winds about the tornado center may attain a velocity of more 
than one hundred miles an hour. Tornadoes are most fre- 
quent in the afternoon of hot summer days, and seem to need for 
their development a fairly level land surface; hence we do not have 
them in mountain regions, nor do they occur upon the Pacific 
coast. The broad, level Mississippi Valley seems best suited of all 
places in the United States for their development. 

Weather Service. — For the purpose of a more thorough study of 
the weather, and more accurate prediction of weather changes, the 
United States Government has established a weather service ex- 
tending to all settled parts of the country. This service, which is 
the work of the Weather Bureau, a division of the Department of 
Agriculture, has its central office in Washington, D. C. Its corps 
of observers, paid and voluntary, to the number of more than 
three thousand, are distributed throughout the country. Regular 
observations of the weather are made at more than two hundred 
stations, as nearly as possible at the same instant, 8 o'clock a. m. 
and p. m. 75th meridian time, and are reported by telegraph to 
the central office at Washington, and to each other. The most 
important observations are: pressure; temperatures, current, maxi- 
mum and minimum; direction and strength of the wiiid; amount and 
kind of precipitation during the past 24 hours, and percentage of 
cloudiness. 



146 



PHYSIOGRAPHY 



Weather Maps. — When these data are collected and plotted on 
a map of the United States the result is a weather map. The daily 
weather map is published at the central office in Washington, and 
also at one or more sub-stations in every State. It not only sets 
forth the weather conditions existing at the time of observation, 
but it also serves as a basis of prediction of the weather for the 




Fig. 68. — Weather Map 
Isotherms, dotted lines, drawn for every 10 degrees; and isobars, unbroken lines, drawn for every 
tenth of an inch. Line of arrows indicates the ordinary path across the U. S. of this type of 
low. Such lows usually advance at the rate of about 30 miles an hour. 

24 or 36 hours following. Each local map supplements the general 
prediction for the entire country with a forecast for the particular 
locality. 

To be of value for purposes of forecasting, weather maps must 
be distributed the day issued, since weather conditions are con- 
stantly changing. 

Since our weather is mainly of cyclonic control, and since the cyclonic 
disturbances move eastward across the country, the weather map as a 
basis of weather prediction is of more value to the eastern than to the 
western part of the country. On the Pacific coast it is of little value, 



WEATHER AND CLIMATE 147 

since there are few stations further west to report coming changes. 
With further extension of wireless telegraphic service, the value of the 
weather service to our western coast will be correspondingly enhanced. 

Value of Weather Predictions. — Every observer is familiar with 
the daily and seasonal changes of temperature; also with the fact 
that there are other almost equally important changes that are 
irregular in their occurrence. More and more people are learning 
to appreciate the relation of these unperiodic changes of the weather 
to the eastward march of cyclonic disturbances, and to appreciate 
the great value of our weather predictions. Each year brings a 
wider use of these predictions, and a more general rejection of 
the predictions of charlatans who make year-long forecasts. 

Among the first to realize the benefits of our weather forecasts 
were the shipping interests of our southern and eastern coasts and 
of the Great Lakes. Not infrequently censuses have shown that 
marine property to the amount of more than $25,000,000 has been 
held in port because of storm warnings issued. Few masters of 
vessels now leave port without knowing the latest forecast of the 
weather. 

Shippers of perishable goods are also interested in weather pre- 
dictions. Estimates from shippers place the value of property 
saved by the warning of the cold wave of January 1st, 1898, at 
nearly $5,000,000. Farmers, planters, truck growers and fruit 
growers are interested in being forewarned of changes in the 
weather, especially when these changes mean destructive winds, 
floods or frosts. With the wide extension of the use of the tele- 
phone and of rural mail delivery is coming a wider use and appre- 
ciation in all fields of the great value of weather predictions. 

It is our confident belief that with a more extended field of observation, 
and a better knowledge of upper air conditions, the present practical 
limit for safe predictions of 36 hours may be considerably increased. 
By means of kites and balloons the upper air is being explored. 

Weather Signs and Proverbs. — There are two distinct classes of 
weather signs. The first are based on century-long observations 
by those whose occupations have led them to observe closely 
weather changes; the second class includes a mass of superstitions 



148 PHYSIOGRAPHY 

that have been strangely preserved and transmitted. The signs of 
the first class have usually found expression in trite sayings that 
have come to be known as weather proverbs. As an aid to memory 
these proverbs are commonly expressed in rhyme. 

Weather proverbs are usually of only local application, though 
many are world-wide. When local, in order to appreciate them, 
one must be acquainted with the local conditions. 

"Rainbow in the morning, sailors' warning; Rainbow at night, sailors' 
delight," is a proverb that is true only in those regions where cyclonic 
storms move eastward. If the rainbow is seen in the morning, the storm 
center is apt to be westward, and its further progress will bring it nearer. 

"Mackerel scales and mares' tails, Make lofty ships carry low sails," 
is applicable the world over. The long, wispy clouds called "mares' 
tails," and the sky flecked with cirro-cumulus clouds, and known as a 
"mackerel sky," are the result of the high-level overflow of air in front 
of a cyclone. Consequently they presage a coming storm. "Mist ris- 
ing o'er the hill, Brings more water to the mill" the world over. 

Climatic Controls. — Since climate is but average weather, those 
conditions which control weather likewise control climate. The 
most obvious, and perhaps the most important, climatic controls 
are: latitude, height above sea level, distance from the sea, posi- 
tion with reference to mountain ranges, and with reference to 
prevailing cyclonic paths. 

Although climate is defined as the average condition of the air 
with reference to the various climatic elements, it does not follow 
that where these averages are (he same the climates are alike or even 
similar. 

New York City and San Francisco have about the same average 
temperature for the year, but New York has hot summers and cold 
winters, whereas San Francisco has equable temperatures through- 
out the year. The central Mississippi Valley has about the same 
annual rainfall as the coast of California; yet in the interior the 
rains are distributed through the year, while on the coast they are 
confined to the winter months. 

Of vastly more importance than averages are the extremes of 
climatic conditions, and the distribution of these conditions 
through the year. 



WEATHER AND CLIMATE 



149 




ISO PHYSIOGRAPHY 

Climatic Zones. — Temperature being the most important cli- 
matic element, and depending, as it does, mainly upon latitude, 
the earth may be divided into east-west zones, each of which 
furnishes a distinct type of climate. Within any zone there may 
be considerable variation from the type, yet there is sufficient 
similarity to justify the division into zones. 

The customary division, whereby the zones are bounded by 
parallels, gives us light zones rather than climatic zones; there- 
fore, the Tropics and Polar Circles are not boundaries for torrid, 
temperate, and frigid climates. A more reasonable boundary is 
the isotherm. It has been suggested that the average annual iso- 
therm of 68° F. be taken as the poleward boundary of the torrid 
zone, and the summer isotherm of 50 F. as the poleward boundary 
of the temperate zones. 

The temperature of 68° F. is about the temperature we desire 
for our houses in winter, and the temperature necessary for so- 
called tropical plants; and a temperature of 50 F. is necessary 
for trees and for the maturing of the hardier cereals. Warm tem- 
peratures during the growing season are more important than low 
temperatures during the dormant season. 

The temperate zone is the widest zone, and wider in the northern 
than in the southern hemisphere. This is due to the excess of land 
north of the equator, land being a better absorber of insolation 
than the sea. The frigid zones, or more accurately the polar cold 
caps, have a temperature, even in the hottest month, below 50 ° F. 

The Torrid Zone. — As its name suggests, the most distinctive 
character of the torrid zone is its high temperatures. In every 
part, except where mountains rise into cold altitudes, the daily 
maximum is from 75 ° to ioo° F., and often higher. But the other 
climatic elements vary so widely in this zone as to justify its 
division into three parts: 

1 . The belt of doldrums or equatorial calms is a belt of high tem- 
perature, light and variable winds, and abundant rainfall. The 
almost vertical rays of the sun heat up the lower air in the early 
forenoon and cause rapid convectional currents. These rise by 
noon to such altitudes that their cooling by expansion produces 



WEATHER AND CLIMATE 151 

condensation of their vapor, the formation of clouds, and in the 
early afternoon rain. The rains are thus of almost daily occurrence 
and abundant throughout the year. More abundant rainfall and 
a higher percentage of cloudiness are to be found over the sea than 
over the land, due to higher humidity over the sea. 

The days vary little in length, and twilight and dawn are of 
short duration, owing to the nearly perpendicular position of the 
sun-path to the horizon. 

In this belt occur the dense forests of South America, Africa, 
and the East Indies. 

2. The trades, like the doldrums, have a prevailingly high tem- 
perature, but unlike the doldrums they have little rainfall. Their 
most marked characteristic is the constancy of their winds. These 
blow day and night, winter and summer, with a constancy of both 
direction and strength equaled in no other zone. In the northern 
hemisphere they are northeast winds, and in the southern hemi- 
sphere southeast. They run to the doldrum belt, where the air 
rises. 

On land the climate of the trades depends upon the direction of 
slope, the eastward and westward slopes having unlike climates. 
The winds being forced up the eastward slopes may yield abun- 
dant rainfall, as upon the eastern coasts of Central America, 
Brazil, Africa and Australia; whereas the descending winds upon 
the westward slopes yield little or no rainfall, as shown by the 
dry western coasts of these countries. 

The eastward and northeastward slopes of the mountainous 
islands of the West Indies and the Hawaiian group have abundant 
rainfall, and are heavily forested; whereas the southwestern slopes 
have deficient rainfall, and in some cases are almost desert. 

Any low-lying land area, island or continent, under the trade 
winds, is apt to be desert because of its slight rainfall. The great 
deserts of Africa and Australia are trade-wind deserts. The winds 
moving toward the equator are warmed, and their capacity for 
water vapor is increased, consequently they not only yield little 
rainfall, but they also absorb the moisture of the regions over 
which they blow. 



J 52 



PHYSIOGRAPHY 




Fig. 70. — Winds of the Atlantic Ocean for January 
Note the well-developed low pressure area over the northern ocean, and the equally well- 
developed high pressure area over the southern ocean. Account for this. Long arrows indicate 
steady winds, and heavy or double arrows indicate strong winds. 



WEATHER AND CLIMATE 



153 




Fig. 71. — Winds of the Atlantic Ocean for July 
Observe that the low in the northern ocean has disappeared and a well-developed high appears 
in the belt of horse latitudes. Find a reason for this. The high continues, though weakened, 
over the southern ocean. 



154 PHYSIOGRAPHY 

3. With the shifting of the wind belts, regions near the border 
line of the doldrums and trades lie alternately under these belts. 
Such regions have seasonal changes of climate, and are known as 
monsoon belts. If near the equator they have two distinct wet 
seasons, when under the doldrums, and two dry seasons when under 
the trades. 

While these transitional or monsoon belts extend around the 
earth, they are most pronounced where, as in India, the monsoon 
winds are reenforced by the continental winds, owing to the great 
continental mass lying poleward from them. When the south- 
west monsoon blows over India the "hooked" southeast trades 
are strengthened by the continental in-draft toward central Asia; 
and when the northeast monsoon blows, the normal northeast 
trades are strengthened by the outflow from the cold continent to 
the north. The southwest monsoon is much stronger than the 
northeast. Why ? 

Similar monsoon winds are observed upon the coasts of South 
America, Africa, and Australia. 

The Temperate Zone. — Poleward from the torrid zone in each 
hemisphere lies the temperate zone. It comprises about one-half 
of the earth's surface, and shows the greatest Variety and range 
of climatic conditions. Lying mainly under the westerlies, its 
weather and climate are mainly cyclonic in character, thus ac- 
counting for its variability of climate. To appreciate the climate 
of any part of this belt it is more necessary to know extremes 
than averages. In northeastern Siberia, for example, an extreme 
range of temperature during the year of over 200 F. occurs, the 
average temperature being about zero. 

In general the variability of climate in the temperate zone is 
greater over the land than over the sea, and greater in the north- 
ern than in the southern hemisphere. 

Divisions of the Temperate Zone. — On this account it is desir- 
able to divide the temperate zones into: 

1. Ocean and land areas; 

2. Northern and southern belts; 

3. Eastern coasts, interiors, and western coasts. 



WEATHER AND CLIMATE 155 

1. The ocean areas of this zone have smaller range of temperature, 
more rainfall and cloudiness, and stronger winds than the land areas. 
The oceans, being low pressure areas in winter, have their greatest rain- 
fall at that season; whereas the continents, on the whole, receive their 
greatest amount of rain in summer. 

2. Because of its large water area the climate of the south temperate 
belt is most equable of all the regions in the westerlies. The winds, 
toward its southern edge, have high velocity and blow with great regular- 
ity; and westward-bound ships around Cape Horn are delayed by these 
head winds. 

The north temperate zone has a climate that has been likened to "A 
crazy quilt of patches," so changeable is it in its various parts. The 
excess of land with its varying altitudes, in this zone, breaks up whatever 
tendency there might be toward uniformity in climatic conditions. 

3. The eastern coasts, though tempered by their adjacence to the sea, 
lying to leeward of the continents, partake of the variability of climate 
characteristic of the land. The succession of lows and highs that march 
eastward across the continents carry to the eastern coasts continental 
conditions. Winds and rains are the result of passing lows and highs; 
and rainfall is slightly more abundant in winter, when the winds from 
the sea blow upon the colder land, than in summer. 

The climates of the eastern coasts are to a slight extent modified by the 
warm and cold ocean currents that follow them. The eastern coast of 
the United States as far north as Cape Cod is to some extent warmed in 
winter when the wind blows from the southeast over the warm Gulf Stream; 
and the eastern coast of North America north of Cape Cod is cooled in 
summer, in like manner, by winds from the cold Labrador Current. On 
this account we find summer resorts on the coast of New England, and 
winter resorts from Florida northward to New York. 

In a similar manner the Japan Current affects the climate of south- 
eastern Asia; while northward from Korea the coast is cooled by winds 
from the cold Kamchatka Current. 

In the southern hemisphere the eastern coasts of all continents are 
warmed by winds from warm currents. 

The interiors of the continents show greater extremes of temperature 
and rainfall than do the coasts. The summers are warmer and the 
winters colder than the latitude justifies. The winds and rains are 
cyclonic, and the rainfall is most abundant in summer, when the lands 
are great centers of low pressure, with inflowing winds. If the region is 
plateau it is characterized by slightly lower temperature and less rainfall 
than if it is plain. 

The western coasts have the benefit of the tempered westerlies coming 
from the ocean. This gives an evenness of temperature throughout the 



156 PHYSIOGRAPHY 

year not found upon the eastern coasts of the temperate zone. Rain is 
both more abundant and more markedly concentrated in the winter 
months than it is upon the eastern coasts, because in winter the colder 
land chills the moist winds from the ocean. Winter fogs are most 
frequent on this coast. 

The northern and southern hemispheres differ widely in the climates 
of their western coasts as a result of ocean currents. Alaska and north- 
western Europe are warmed many degrees above the normal for their 
latitude by winds from the great warm drifts in the Pacific and the At- 
lantic oceans, while Chile, western Africa and western Australia are 
cooled as a result of the branches along these coasts from the cold Ant- 
arctic Drift. 

The climatic influence of warm and cold currents is much greater on 
windward than on leeward coasts. In the westerlies the windward 
coasts are the western coasts, whereas in the trade belts the windward 
coasts are the eastern coasts. Winds blowing over ocean currents acquire 
the temperatures of those currents, which they carry to the lands. 
Winds blowing across continents tend to carry continental conditions 
to the eastern coasts in the westerlies, and to the western coasts in the 
trades. 

While the chief effect of the warm currents in the northern oceans is 
to temper the cold of the western coasts in high latitudes, a less marked but 
noticeable effect is to temper the heat of the western coasts in low latitudes, 
upon the return of the currents toward the equator. Southern California, 
western Mexico, and northwestern Africa are thus made cooler. 

Between the temperate and torrid zones lies a transitional zone 
of high pressure and descending air currents. This belt shifts 
during the year with the northward and southward movement of 
the sun. These are, therefore, monsoon belts, places in them having 
alternately the climate of the trades and of the westerlies. Rain- 
fall is scant, owing to the increased temperature, due to compres- 
sion, of the descending currents; and the winds are never strong, 
since this belt is the starting point of both the trade winds and the 
westerlies. 

In general, seasons in the temperate zone are based chiefly upon 
temperature, whereas those of the torrid are based upon rainfall. 
The transition seasons of spring and fall, though well marked in 
the middle of the temperate zone, disappear toward the poleward 
and equatorward edges. Mountain ranges with a north-south 
trend induce rainfall upon their western slopes, while arid or des- 



WEATHER AND CLIMATE 157 

ert regions are apt to lie to eastward of them. This is the reverse 
of conditions in the trade wind belts, where the eastward are the 
rainy, and the westward the dry slopes. 

The Polar Cold Caps. — The polar areas, though called the frigid 
zones, are not belts, but an area about each terrestrial pole in- 
cluded within the summer isotherm of 50 F. The polar cold cap 
is much more extensive in the southern than in the northern 
hemisphere, possibly due to the great glacial ice-sheet covering 
the Antarctic continent, or to aphelion winter. In the Antarctic 
regions the boundary isotherm reaches in some places to latitude 
55, while in the Arctic regions it crosses and re-crosses the Arctic 
circle. 

These regions are rightly named frigid, the chief characteristic 
of their climate being freezing temperatures for most of the year. 
At no time is the temperature high. The lands, for the most part 
covered by ice, are frozen deserts. It is only where there are slopes 
favorably inclined to the sun that the snow melts and the soil is 
sufficiently warmed and drained to permit plants to grow. Even 
here the temperature is too low for trees to thrive, and mosses and 
lichens are the chief growth. Temperatures of —&3 F. have been 
reported. 

The winds over the polar cold caps are cyclonic, and the cyclones 
are probably driven cyclones. The winds are often burdened with 
fine dry snow, which covers all land surfaces for most of the year. 
Over the stretches of frozen plain these winds sweep with great 
violence, and are comparable to the blizzards of winter climates 
in temperate latitudes. 

Precipitation, which on the whole decreases toward the poles, is 

• deficient here. It is mainly in the form of snow, there being some 

regions where rain probably never falls. Though the precipitation 

is light, yet it probably exceeds evaporation over the region, the 

excess being imported by the westerlies from lower latitudes. 

Since more snow falls than melts, the land areas of Greenland 
and the Antarctic continent become covered with a glacial ice- 
sheet. When the glaciers reach down to the sea, great blocks of 
ice break off and float away to lower latitudes, as icebergs. 



158 PHYSIOGRAPHY 

Continental and Marine Climates. — The interiors of all conti- 
nents are marked by great variability of temperature. This 
variability decreases toward the coasts, being least upon windward 
coasts. In strong contrast with the climates of continent in- 
teriors is the climate of islands, which is practically that of the 
surrounding sea. 

The sea is less heated and less cooled than the land, under the 
same conditions of insolation. It is a better reflector and trans- 
mitter than the land, hence less insolation is absorbed to warm 
the surface layers. The specific heat of water is higher than that 
of the land, hence a given amount of heat will not raise the tempera- 
ture of the water through as many degrees as it will the same mass of 
land. Much of the heat absorbed by water is used up in the me- 
chanical work of evaporation, leaving just so much less for raising 
the temperature; while on land, the amount of evaporation is lim- 
ited by the amount of moisture brought up to the surface by capillarity, 
and nearly all of its absorbed heat is used to increase its tempera- 
ture. Then there are currents in the ocean that distribute the 
heat over great distances; and there is a greater percentage of 
cloudiness over the sea than over the land. 

All of these variables combine to give to places surrounded or 
bordered by the sea peculiarly equable climates as compared with 
continent interiors in the same latitude. The one climate we call 
marine, the other continental. 

Mountain Climates. — As we ascend a mountain, in any lati- 
tude, all the climatic elements change from those prevailing at 
the mountain's base. The temperature, as we have seen, falls at 
the rate of about i ° F. for every 300 feet of ascent ; the absolute 
humidity decreases, while the relative humidity generally increases 
up to a certain altitude, depending upon the latitude, after which it 
also decreases; precipitation increases for a time, as we ascend, then 
gradually fails; and with increasing altitude the winds increase in 
strength and constancy. 

On the whole, all of the climatic elements become more constant 
with increase of altitude, and this equability is the most marked 
characteristic of mountain climates. 



WEATHER AND CLIMATE 159 

The windward and leeward sides of mountains, and particularly 
of mountain ranges, are apt to present very different types of 
climate. The windward, owing to ascending and cooling currents, 
has an excess of rainfall, while the leeward, with descending and 
warming winds, is apt to be dry. If in the trades, the eastern 
slopes receive the rainfall, whereas in the westerlies the eastern 
slopes are dry. 

The desert of southern California in the United States, and of 
Atacama in northern Chile, are examples of deserts to leeward of 
mountains in the westerlies, and the arid western coasts of Mex- 
ico and Peru lie to leeward of mountains in the trade wind belts. 
The southern slopes of the Himalayas receive most of their rain- 
fall while the southwest monsoon blows; and the northern slopes 
of the Atlas mountains in northern Africa are the windward and, 
therefore, the rainy slopes. 

QUESTIONS 

1. Why are weather changes more pronounced near sea level than at 
higher altitudes? Inland, than near the coast? In high, than in low 
latitudes? 

2. Why do cyclones endure longer at sea than on the land? 

3. Why should rainfall be more abundant behind than in front of the 
cyclone centers in the trade wind belts? 

4. What is the direction of the wind in front of, and behind, tropical 
cyclones, both in the northern and the southern part of the torrid zone? 

5. Why are thunderstorms more frequent in summer than in winter? 

6. Why do tornadoes generally occur in the afternoon? And why do 
people in the United States not seriously fear a threatening cloud in the 
north, but do, one in the southwest? 

7. Classify the weather proverbs you know with respect to their scien- 
tific basis. 

8. Why do the continents in temperate latitudes have their greatest 
rainfall in summer? 

9. Why is the climate of the north temperate zone more variable 
than that of the south temperate? 

10. Why do the interiors of continents show the greatest extremes of 
climate? 

11. Why do the eastern coasts in the trade wind belts have more 
equable climates than the western coasts of these belts, whereas the re- 
verse is true in the belts of prevailing " westerlies? " 



CHAPTER XIV 
CLIMATE OF THE UNITED STATES 

Climatic Regions. — The United States is situated in the zone 
of prevailing westerlies, hence its climate is chiefly of cyclonic 
control. However, its wide range in latitude, its great variation 
in distance from the sea, and the difference in altitude of the various 
parts give to different sections sufficiently characteristic climates 
to justify their separate consideration. 

Minnesota and Maine, because of their higher latitude, have 
lower average temperatures than Louisiana and Florida; Kansas 
and Nebraska, lying near the center of the continent, have greater 
ranges of temperature and less rainfall than northern California 
and Maryland; and Denver, in the foothills of the mountains, has 
a more equable climate than St. Louis, in about the same latitude 
but at a lower level. 

Based upon these three conditions governing climate, latitude, 
distance from the sea, and altitude, it has been suggested that the 
United States be divided into north-south climatic regions. Some 
of these regions vary considerably from north to south. 

The Pacific Coast Region. — This zone extends inland from the 
Pacific coast about 200 miles to the backbone of the Sierra Nevada 
and Cascade Mountains. Like all regions situated to leeward of 
an ocean, it is characterized throughout by an equable climate. The 
isotherms, instead of having a roughly east-west trend, as is their 
usual habit, run almost parallel with the coast. The continuation 
of the Japan Current, the North Pacific Drift, cooled during its 
long journey through north Pacific waters, in its southward flow 
washes the entire length of coast of this region. The influence of 
the winds from over this current is perhaps to increase the temper- 
ature slightly, of the State of Washington, above the average for 



CLIMATE OF THE UNITED STATES 161 

that latitude, but to lower the temperature decidedly, of southern 
California. Frost seldom occurs here in the lowlands. 

In strong contrast with this sameness of temperature through- 
out its north-south extent is its wide difference in annual rainfall. 
The westerly winds come from the Pacific, moisture laden at 
all seasons. In summer they blow upon land surfaces warmer 
than themselves and, therefore, yield no rain until they begin to 
rise up the mountain slopes. In winter the cooler land induces 
rainfall, even over the lowlands, thus making the winter rains 
the most marked characteristic of the Pacific Coast climate. 

Upon the mountain slopes the rainfall is abundant throughout 
the year, though most abundant in winter. There we find the 
giant trees and dense forests. On the coast of Washington, where 
the high mountains lie near the sea, we find the greatest rainfall 
of the United States, over ioo inches; whereas in southern Cali- 
fornia, with its coastal plain, and its nearness to the high pressure 
calms, it is less than 10 inches. 

The cultivated lowlands at the south are parched during the 
growing season, and but for their nearness to the mountains, which 
makes irrigation possible, these lowlands would be of but little 
value. As it is, they are among the most valuable cultivated lands 
in the United States. 

It is claimed for these lands that they are peculiarly adapted to 
the production of fruits, inasmuch as the fruits grow and ripen in 
sunshine, thus giving them higher color and superior flavor. 

Along the coast fogs are common, especially in winter. Severe 
storms are almost unknown, thunder being rarely heard upon the 
coast. Upon the mountain slopes thunderstorms break, and the 
lightning flashes are seen, though at distances from the coast too 
great for the sound of the accompanying thunder to carry. 

The Plateau Region. — This region embraces the high plateau 
lands lying between the Sierra Nevada and Cascade Mountains on 
the west and the Rocky Mountains on the east. Its most marked 
characteristic is its dryness. Lying as it does to the leeward of the 
Sierras, the descending winds on the eastern slopes of these moun- 
tains yield little rain. It is only after they have crossed the 



162 PHYSIOGRAPHY 

greater part of the region, and begin their ascent of the western 
slopes of the Rockies, that rain is induced. Occasional cyclonic 
storms yield some rain, but over most of the region the rainfall 
is insufficient for agriculture, without irrigation. It varies from 
20 inches in Washington to 3 inches in Arizona. The greater part 
of this region is too remote from the mountains to permit of irri- 
gation, and must, therefore, remain arid and unproductive. 

The skies over the plateau region are prevailingly clear, conse- 
quently the daily range of temperature is excessive. The winters 
are cold, and the summer days extremely hot. Cold winter cy- 
clones sweep down from the north ; and it has been suggested that 
the hot desert region about the head of the Gulf of California is 
the birthplace of most of our southwest summer cyclones. 

Rainfall is nowhere in the region sufficient to support heavy 
forests. At the north, where most abundant, it falls mostly in 
winter, the growing season being almost without rain. Owing to 
the deep and retentive soil, so fine-grained and homogeneous that 
it brings capillary water from unusual depths, this part of the 
region yields abundant wheat harvests. Apples and other tem- 
perate latitude fruits are grown where irrigation is possible. 

The chinook winds, which sink down the mountain slopes, 
warming as they advance, keep the narrow mountain valleys free 
from snow. On this account these valleys are much sought by 
both wild and domestic animals for winter grazing grounds. 

The Great Plains. — This name is applied to the region of east- 
ward sloping lands from the Rocky Mountains to about the 
meridian of ioo° W. It grades imperceptibly eastward into the 
next climatic region, and is characterized over most of the region 
by the cold winters and hot summers, typical of continental inte- 
riors in this latitude. 

Rainfall, which increases eastward with increasing distance from 
the mountains, is in the main insufficient for agriculture, unaided 
by irrigation. Much of the region is capable of irrigation, from 
streams or artesian wells, and by this means is becoming increas- 
ingly valuable. The rainfall is insufficient for forests, but it suffices 
for the growth of abundant and nutritious grasses. These are the 



CLIMATE OF THE UNITED STATES 163 

great natural grazing grounds of the United States. Before the 
advent of the white man vast herds of buffalo roamed these plains, 
but disappeared with the march of civilization westward. In their 
stead came herds of cattle and flocks of sheep, and that typically 
western product, the "cow-boy." 

The seasons are variable in the extreme. Occasional abundant 
harvests are gathered, only to be followed by one or more seasons 
of disastrous failure. With wider adoption of the methods of "dry- 
farming," much more of the Great Plains region will be devoted to 
agriculture. 

The rains and winds of the region are wholly determined by the 
passage of highs and lows. The rainfall is distributed through the 
year, though slightly in excess in summer, and nowhere exceeds 
20 inches. 

This region is the continuation of the great Arctic plain, which 
extends unbroken southward past Hudson Bay. With no east- 
west mountain range to interrupt, it is swept over by the winter 
cyclones from the north, which sometimes reach even to the Gulf 
of Mexico before turning to their final northeastward course. 
Owing to the level and prairie character of the region, wind veloc- 
ities are often excessive. 

The Central Prairie Lands. — As already stated, this region is a 
continuation eastward of the Plains region, there being no natural 
boundary between them. It is bounded eastward by the Missis- 
sippi River. 

The Central Prairie Lands differ from the Great Plains chiefly 
in having a more abundant rainfall, 20 inches or more. This is 
everywhere sufficient for agriculture, and increases southward. 
On the coast of Louisiana it is 60 inches, due to the in-draft of 
warm winds from the Gulf, toward lows crossing the region far- 
ther north. 

The climate, though typically cyclonic, is not so extreme as 
farther west. At the south the influence of the Gulf in tempering 
both the cold of winter and the heat of summer is marked. The 
annual range of temperature is 160 in North Dakota, whereas 
it is but half that on the Gulf coast. 



1 64 PHYSIOGRAPHY 

Over most of this region the rainfall, 30 inches, is sufficient to 
support forests, and their absence over much of the region has not 
been satisfactorily explained. Forests border practically every 
stream of the region. 

Various explanations of the absence of forests in this region have been 
proposed. The one which perhaps has received widest acceptance is the 
destruction of the forests by fires. Inasmuch as attempts to extend the 
forests have not been successful it would seem that perhaps the explana- 
tion of the absence of forests is to be found in the character of the soil, 
and deeper deposits, which are often glacial clays. 

The great body of the more productive agricultural lands lies in 
this division, and here most of the staple food products are grown. 

The winds are variable in direction, though northerly winds 
prevail at the north and southerly winds prevail at the south. 
Owing to the greater interruption by forests the winds do not here 
attain the average strength of the winds upon the Great Plains. 

This climatic region is visited by a greater number of destructive 
wind storms than any other region of the United States. Torna- 
does begin to occur in the Gulf States in February, though most 
frequent from April to September. Their time of earliest occur- 
rence is later the farther north we go. 

The Western Appalachian Slope. — This region embraces the area 
extending from the Mississippi eastward to the axis of the Appala- 
chian Mountains, and from the Great Lakes southward to Ten- 
nessee. At the north the Great Lakes temper both the heat of 
summer and the cold of winter, so that the climate, though conti- 
nental, is not so extreme as in the regions between the Mississippi 
River and the Rocky Mountains. 

The westward trend of the Appalachians, while protecting the 
Gulf slopes to the southward from cyclones originating in the west, 
also protects the climatic region to the northward from the frequent 
tropical storms that come up from the West Indies. These tropical 
cyclones rarely cross the mountain barrier of the Appalachians. 

The rainfall of the region exceeds that of the northern division 
of the Prairies for two reasons: there is a greater water surface 
adjacent, to yield vapor, and the prevailing westerlies are moving 



CLIMATE OF THE UNITED STATES 165 

up the slopes. The rivers are, therefore, numerous and strong, 
and more evenly and abundantly supplied than are those of the 
Prairie region. Though well distributed through the year, the 
rainfall is more abundant in summer than in winter. This is in 
part due to the greater absolute humidity of the air in summer, 
and in part to the more frequent passage of lows having their 
origin in the southwestern part of the United States. Winter 
cyclones more commonly originate in the northwest, and are not so 
likely to be accompanied by precipitation. The precipitation in 
winter is chiefly in the form of snow, especially in the lake region. 

The winds are cyclonic, but less strong than in the Prairie re- 
gion, because of the generally forested character and greater un- 
evenness of the lands of this region. 

The Atlantic-Gulf Slope. — This slope, extending from Maine to 
Louisiana, presents a great variety of climate. Being near the 
sea, neither the extreme cold of winter nor the heat of summer of 
regions farther inland is felt; but, being to leeward of the continent, 
the equalizing influence of the sea is not nearly so great as upon the 
Pacific slope. At the north the winters are cold, and the summers 
cool; while at the south the winters are temperate, and the sum- 
mers, owing to the excessive humidity, are oppressive, though not 
so warm as farther north inland. 

Ocean currents are important factors in determining the climate 
of the Atlantic Coast. The cold Labrador Current, coming down 
the New England coast, makes that coast colder as far south as 
Cape Cod, while the Gulf Stream influences the climate of the 
coast from Florida northward to Cape Cod. 

Rainfall, abundant throughout this climatic region, increases 
generally toward the south, where it is more than 60 inches. It is 
well distributed throughout the year, though for the greater part 
of the region it is most abundant in the fall. Toward the south 
the maximum fall is later, in southern Florida being most abundant 
in winter, when the westerlies prevail. However, the southern 
Florida rains are not of the Pacific Coast type of winter rains, 
being mainly due to passing lows, and not to forced ascent over 
cold lands. 



1 66 



PHYSIOGRAPHY 




CLIMATE OF THE UNITED STATES 



167 



\ I ■ - - V 



■ / 



Mf-A 




1 68 PHYSIOGRAPHY 

South of Cape Hatteras the coast is often swept by tropical 
cyclones which reach our coast from the southeast, whereas north 
of Hatteras the cyclones are from the west or southwest, and orig- 
inate outside the tropics. Both types of storms move northeast- 
ward, their paths converging, thus giving to New York and Bos- 
ton a greater number of cyclonic storms than to points either 
farther north or farther south. 

Exceptional Conditions. — In many of his activities man is con- 
trolled not so much by usual as by exceptional conditions of 
climate. Our buildings are constructed to withstand the strongest 
wind, and the levees along our rivers to restrain the highest flood. 

As we have seen, profitable agriculture is not so much dependent 
upon the annual rainfall as upon the occurrence of rain during the 
growing season. 

In order to obtain a clear understanding of the climates of the 
several climatic regions of the United States, it is necessary to 
examine maps showing averages for given periods, and maps show- 
ing departures from these averages. 

From the January chart of temperatures is seen the wide difference 
in the winter temperature of 70 at Key West, and of — 5 in North 
Dakota. This difference, while in part due to difference of latitude, is 
to a much greater extent the result of the difference between coast and 
continent-interior conditions. Along the Atlantic coast the change in 
temperature is from 70 at Key West to io° in northern Maine, while 
along the Pacific coast, for the same change in latitude, there is a change 
if only io° in temperature. In the contrast shown by these two coasts 
is seen the difference due mainly to position to leeward of a continent 
and position to leeward of an ocean. 

The July chart of temperatures tells a very different story. No 
longer is the highest temperature found at Key West, but in Arizona, 
some 8° farther north. This is largely due to the arid character of this 
region, with its prevailingly clear skies. Along the Atlantic slope there 
is a difference of temperature of about 25 between Florida and Maine, 
as compared with 6o° in January, while on the Pacific coast the difference 
for July is about the same as for January, the isotherms, as we see, run- 
ning almost parallel with the coast. The interior of the continent, which 
is colder than similar latitudes upon the coast in January, is now seen to 
be warmer. The isotherms for July bend northward in crossing the con- 
tinent, whereas those for January bend southward. 



CLIMATE OF THE UNITED STATES 1 69 

While the lines of equal minimum temperature follow, in general, the 
trend of the isotherms for January, the lines of equal maximum tempera- 
ture are not so regular. We find the lowest minimum, — 63 , in the 
interior of the continent, near its northern boundary, and the highest 
minimum, 40°, at Key West. Here frost never occurs. The minimum 
temperatures of the Atlantic coast are from 20 to 30 lower than those 
of the Pacific coast in the same latitudes. Here again is shown the 
difference in windward and leeward coasts. 

The tempering influence of the sea is well shown by a comparison of 
the average maximum of the coasts, about 05 F., with that of the in- 
terior, which is about io° higher. The lowest maxima, about oo°, are 
found in the extreme northeast and northwest coastal regions, while 
the highest, almost 125 , occur in the interior desert region of southern 
California and Arizona. Maxima exceeding 105 are common over the 
Great Plains region, but the dry heat of this region is not so oppressive 
as that of the Gulf and Atlantic coasts, where the maxima are 5 lower. 

The range of temperature is the difference between the summer maxi- 
mum and the winter minimum. The greatest range is found in the 
northern interior, whereas the lowest range occurs at Key West. In 
general, range of temperature increases with increase of latitude and 
with distance from the sea. 

The range of temperature along the Pacific coast varies little, being 
only 1 5 greater at Puget Sound than in southern California, whereas 
the Atlantic coast varies in range from 50 at the south to no° in Maine. 
The range of temperature for most of the Gulf coast is about 85 , whereas 
that for Montana is twice as great. 

Freezing Temperatures. — The number of days with average temper- 
ature below freezing varies from none in the Pacific, Gulf and Atlantic 
coast regions northward to Chesapeake Bay, to 165 days in Minnesota 
and North Dakota. Of much greater practical interest to farmers and 
fruit growers, however, are the dates of occurrence of earliest and latest 
killing frosts. 

In the fall, with the lengthening night and increasing slant of the sun's 
rays, there comes a time when the daily minimum falls almost to freezing. 
The passage of a low across the continent then is apt to be followed by 
frosts. These are due to the cold in-draft of north winds at the rear of 
the low. where the sky is clear and the winds light. 

The date of occurrence of the first killing frost in the extreme north 
central part of the United States is about September first. As the 
winter season marches southward, and toward the coasts, the first kill- 
ing frosts occur later and later in these directions, being as late as 
December 15th in central Florida. 

In spring, when the noon altitude of the sun is increasing, and the days 



170 



PHYSIOGRAPHY 




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CLIMATE OF THE UNITED STATES 175 

are lengthening, there comes a time when the ordinary daily minimum 
ceases to fall below freezing. But for weeks after this condition is 
reached, a passing low, with its cold in-draft of northern winds behind, 
may bring freezing temperatures; and thus the time of latest killing 
frost be made later. At such times falling temperature and clearing 
skies forewarn of frost. 

Since spring marches northward and landward from the coasts, the 
average time of latest killing frosts is earliest at the south; and in any 
latitude, at the coast. It occurs along most of the Gulf coast about 
February first, and is delayed in the extreme northern part of central 
United States until almost June first. 

The absolute date of latest killing frosts is considerably later than the 
average date in all sections, being much nearer March first on the Gulf 
coast, and July first in Minnesota. 

From the accompanying rainfall charts we are able to locate the 
regions of greatest and of least rainfall during the year, as well as 
the more important matter of its distribution in time. For the 
farmer and planter this last is of the greatest importance. 

The least rainfall, three inches, occurs in southwestern Arizona. 
Most of this amount may fall in a single day, or indeed in a few 
hours, during a single thunderstorm. The greatest annual rain- 
fall in the United States, over 100 inches, occurs in northwest 
Washington, and while most abundant in winter, is fairly well 
distributed through the year. The annual rainfall on the Pacific 
coast decreases southward, in central California being but half of 
the maximum in Washington. 

On the Atlantic coast the maximum rainfall is near Cape Hat- 
teras, decreasing northward and southward. 

A rainfall of two to four inches a month during the growing 
season is desirable for agriculture. Occasionally many times this 
amount falls, as much as ten inches being recorded in a single day. 
Such torrential downpours are injurious alike to growing crops 
and to cultivated lands. The soil is washed away, streams are 
flooded and overflow their banks, causing destruction of property 
and life. These heavy downpours are popularly know r n as cloud- 
bursts. 

The recorded rainfall includes snowfall, ten inches of snowfall 
being estimated, when melted, as the equivalent of one inch of rain. 



176 



PHYSIOGRAPHY 





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CLIMATE OF THE UNITED STATES 177 

Distribution of Snow. — Every part of the United States, excepting 
southern Florida and southern California, receives some snowfall. It is 
least at the south, and increases with latitude and altitude. It is more 
than 40 inches in the region of the Great Lakes and in the Rocky Moun- 
tains, and occasional heavy snowfalls occur in the extreme south. A 
fall of 13 inches occurred at Baton Rouge, Louisiana, in 1895, during a 
single storm in February; but such snows usually melt within a day or 
two after falling. 

The greatest annual snowfall in the lowlands of the United States, 
130 inches, occurs in the northern peninsula of Michigan, the moisture 
being supplied from the adjacent lakes. The greatest average annual 
snowfall of the entire country, not including Alaska, occurs in the Sierra 
Nevada Mountains. The moist westerlies from the Pacific, compelled 
to rise in passing over the mountains, precipitate, on an average, 378 
inches of snow at Summit, California. 

The Rocky Mountain region has a heavy annual snowfall, though less 
than the Sierra Nevada and Coast ranges. It is mainly to the melting 
of these snows in the Rockies that the great irrigation projects look for 
their supply of water. The floods in the Missouri and other eastward 
flowing streams with sources in these mountains occur in May and June, 
when the normal rainfall is augmented by the melting snow. 

The snowfall in the northern plains and prairie regions is variable; 
some winters it is excessive, others, light. When abundant in the wheat- 
growing sections a good crop is expected, since the snow serves as a pro- 
tection from the cold, and also leaves the soil in good condition. 

In the lumbering sections of the north, from Minnesota to Maine, 
the profits of the season are directly related to the snowfall, which is 
usually abundant. Little snowfall in these regions means smaller output. 

Number of Days with Precipitation. — The number of rainy or snowy 
days during the year varies widely in different sections of the country. 
In general, it is least in the interior, and increases toward the coasts; 
and is greater in the north than in the south. The greatest number, 
180, occurs in northwest Washington; then follows the Great Lakes 
region, with 170 days. In the southwest desert region the number falls 
to 13. For most of the agricultural sections the number varies from 100 
to 140. Forty consecutive rainy days are reported in northwestern 
United States, and 150 days of consecutive drouth in the arid region of the 
southwest. The more equable the distribution of rainfall during the 
year the less the liability to long-continued rains or drouths. 

Humidity. — The absolute humidity of the air is greater in southern 
United States than in northern; is greater in summer than in winter, 
and greater near the coast than in the interior. 

The relative humidity is on an average lowest on the Colorado plateau, 



178 PHYSIOGRAPHY 

where it is about 40%, and highest on the eastern and western coasts, 
about latitude 40 N., where it is about 80%. In the Gulf region it is 
about 75%, and approximately the same in the region of the Great 
Lakes, although, as a rule, continent interiors favor low relative humidities. 
The percentage of cloudiness agrees well in winter with the relative 
humidity, but in summer one of the areas of greatest cloudiness is over 
the Colorado plateau, where the average relative humidity is low. This 
is probably due to the strong convectional currents set up during the 
summer season, the air rising to sufficient heights for saturation. 

Winds. — As before stated, the winds are stronger upon the 
coasts, and over the prairie regions than over forests and moun- 
tainous regions. For most of the country the season of strongest 
winds is spring; and the month of weakest winds, August. 

Aside from tornadoes and hurricanes, during which, for a few 
seconds, the velocity of the wind may considerably exceed 100 
miles an hour, the strongest winds are about 70 miles an hour 
inland, and 90 miles an hour on the coasts. 

Though the direction of the wind is variable in all parts of the 
United States, in valleys there is a decided up or down the valley 
tendency in wind direction. On the coasts, in winter, there is a 
predominance of land winds. This is especially true of the Gulf 
and Atlantic coasts. On the Pacific coast the meeting of the land 
winds and the prevailing westerlies produces " along-shore " winds. 
In summer the conditions upon the Atlantic and Pacific coasts are 
reversed. The Pacific now has strong ocean winds, while the in- 
blowing winds upon the Atlantic coast are met by the westerlies, 
producing "along-shore" winds from the southwest. 

The winds which bring cloudy weather and precipitation vary 
with the section. They are generally winds blowing from the 
nearest great water body. On the Atlantic and Gulf coasts, and 
over most of the interior of the United States, they are east and 
southeast winds, while on the Pacific coast they are generally 
southwest winds. In winter, in the northern section of the United 
States, snow often accompanies winds from a northerly direction. 
Winds from southerly directions, in front of the low, bring higher 
temperatures, and yield rain, while the colder winds in the rear 
yield snow. 



CLIMATE OF THE UNITED STATES 179 

QUESTIONS 

1. Why is the rainfall of the Pacific coast so much greater in Wash- 
ington than in southern California? And why are the rains at the north 
less distinctly winter rains than those at the south? 

2. Why should thunderstorms be practically unknown upon the 
Pacific coast? 

3. Are the Sierra Nevada or the Rocky Mountain ranges more respon- 
sible for the arid climate of the Great Basin? Why? 

4. Why does the Atlantic coast have so much greater variation in 
temperature than the Pacific? 

5. From what direction do storms in your section usually come? 

6. What direction of wind is usually coldest? 

7. What direction of wind is most apt to bring snow in winter and rain 



in summer 



8. How does knowledge of your climate concern your daily life and 
occupation? 



PART III 

THE SEA 



CHAPTER XV 
GENERAL CHARACTERISTICS OF THE SEA 

The Relation of the Sea to the Land. — Most of the phenomena 
connected with the wearing away of the land, with moderating the 
climate, and even with the existence of life itself, depend in large 
measure upon the sea. The source of the water supply for the 
land is the sea; and the streams with their sediments from the land 
return to it. 

The sea is a great international highway, and plays an impor- 
tant part in the commerce of the world. It is no longer a barrier 
between countries. The great steamships are little affected by 
storms at sea. Being equipped with wireless telegraph instru- 
ments, ships communicate with each other at sea and with land 
stations, thus removing the isolation that was formerly experienced 
in crossing the great oceans. Countries are connected by sub- 
marine cables so that news is sent and business transacted be- 
tween nations separated by oceans, almost as easily as between 
different parts of the same country. The digging of canals across 
isthmuses tends to change routes of travel and commerce at sea. 
The Suez Canal has had a far-reaching effect on trade in the Old 
World, and the Panama Canal will influence trade routes in the 
New. 

The surface of the sea is commonly regarded as having a very 
nearly uniform level, known as the " level of the sea," from which 
land elevations and sea depressions are measured. The sea is 
drawn toward and upon the continents that surround it, especially 
when large mountain masses are situated near the coast, so that 
sea level cannot be of uniform curvature. The actual deformation 
of the ocean level in different parts of the earth due to this cause 
has been estimated to amount to several hundred feet. On the 



1 84 PHYSIOGRAPHY 

coast of India, owing to the attraction of the great Himalaya 
Mountains, the water stands much higher than water in mid- 
ocean or water along a lowland coast, such as western Europe or 
that of the eastern United States. 

The extent of the sea has not been constant in ages past and is 
not now a fixed area. Much of the land furnishes evidence that 
it has at some time been covered by the sea, and regions now sea- 
bottom have been land. The great central valley of the United 
States was once sea floor, there being an unbroken stretch of sea 
from the Gulf of Mexico to the Arctic Ocean. On the other hand, 
land along the eastern coast of North America has suffered drown- 
ing. 

Scientific explorations of the sea, made by different governments, 
by societies, and by individuals, from time to time, have given 
us most of our knowledge of the depth of the ocean, its tempera- 
ture, its movements, its deposits, and its life. 

Divisions of the Sea. — The continuous body of salt water called 
the sea, covering about three-fourths of the earth's surface, has five 
divisions, called oceans. The polar circles, the continents, and the 
meridians from their southern points form the boundaries. 

The Pacific is the largest ocean, comprising three-eighths of the 
entire sea area. Its greatest width is about 10,000 miles, in a 
direction east and west along the equator. It is characterized on 
its Asiatic shores by numerous border seas, festoons of islands, and 
many rivers; and on its American shores by high mountain ranges 
parallel to the shore, and few rivers. 

The Atlantic is the second in size, with an area about one- 
quarter of the whole sea surface. It has an average width of 
3,600 miles. The equator divides both the Atlantic and the 
Pacific Ocean into a northern and southern part. The North 
Atlantic, both on the American and the European sides, has 
many seas and bays which give it an irregular shore line. It has 
a wide continental shelf and many rivers. The South Atlantic 
has a more even shore line and few good harbors. 

The Indian Ocean has an outline that is roughly circular. It 
has one-eighth of the total sea area and a diameter of about 



GENERAL CHARACTERISTICS OF THE SEA 185 

6,000 miles. The Indian Ocean is bordered by large seas and bays, 
and a northern and western boundary consisting of very high pla- 
teaus and mountains. 

The Arctic Ocean is an extension of the Atlantic. It has a 
width of about 2,500 miles and about one-thirtieth of the sea area. 
A considerable area of the Arctic is covered most of the year with 
drifting ice. 

The Antarctic Ocean lies within the Antarctic Circle. Within 
this region there is a continent covered with an ice cap thousands 
of feet thick The relative area of land and water in this frozen 
region is at present unknown. 

Distribution of the Ocean Waters. — By holding a globe so 
that the greatest expanse of water is seen, the island of New Zealand 
will be found to be near the center of the water hemisphere, or 
what might be called the water pole of the earth. London, 
England, will be found to be nearly opposite, and the center or 
pole of the land hemisphere. 

Depth. — The greatest known depth of the ocean is 31,614 feet, in 
the Pacific, near the Ladrone Islands. This depth is a little greater 
than the height of the highest mountain above the sea level. 
Many places in the sea are more than four miles deep, and the 
area of surfaces of the sea floor in deep water greatly exceeds the 
area of high land. The average depth of the ocean is about 2^ 
miles, and the average height of land about half a mile. It may 
be inferred from this that the continental land masses would 
make a small beginning in filling up the deep sea. 

Composition of Sea Water. — The water of the sea is so salt 
and bitter as to be undrinkable. If 100 pounds of sea water are 
evaporated, about 3^ pounds of a whitish powder will remain. 
About three-fourths of this powder is common salt. The bitter- 
ness is due to chloride of magnesia, Epsom salts, gypsum, and 
small quantities of almost every soluble substance known. Sea 
water contains in addition to mineral matter dissolved atmospheric 
gases. Oxygen is more abundant in the water near the surface, 
and the proportion of carbon dioxide increases toward the bottom. 



1 86 PHYSIOGRAPHY 

The oxygen dissolved in the water is being consumed by marine life 
and its supply is furnished by the atmosphere. The amount of 
saltness of the sea varies slightly in different parts of the earth. 
Where evaporation is more rapid, as in the trade wind belts, the 
saltness of the water is greater, since salts are left behind when 
sea water evaporates. When rainfall is abundant, as in the dol- 
drum belt, the sea water becomes less salt and of less density. 
Rivers bring to the sea fresh water which mixes with the salt 
water and makes it of less density. 

Temperature. — The surface waters of the sea are warmest, as 
the water is heated by the sun's rays; and the warmer water being 
lighter than the colder water, remains at or near the surface. The 
temperature varies from about 80 degrees near the equator to about 
29 degrees near the poles. The decrease of temperature with 
increase of latitude is far from being regular, the irregularity 
being largely due to ocean currents which vary in temperature 
from that of the surrounding water. 

The surface waters of the sea are alternately warmed and 
cooled in both hemispheres, depending upon the season of the 
year. At the equator and the poles the seasonal change is slight, 
but in middle latitudes it amounts to several degrees. In the 
latitude of New York the winter temperatures are usually be- 
tween 50 and 60 degrees, and the summer between 60 and 70 
degrees. 

The temperature of water below the surface falls rapidly with 
increase of depth. Even near the equator the temperature at a 
depth of less than half a mile is usually below 40 degrees. At the 
bottom of the deep sea the temperature is generally below 35 
degrees. 

The decrease of temperature with increase of depth is not 
uniform because of the deep circulation of the ocean water. Be- 
cause of currents beneath the surface sometimes warmer and 
sometimes colder, slight irregularities in temperature occur. Sea 
water, when cooled either by cold air or by melting ice, tends to 
sink. The great supply of cold water from the polar regions 
creeps along the bottom of the sea and is the cause of the low tem- 



GENERAL CHARACTERISTICS OF THE SEA 187 

perature in the equatorial as well as in the temperate and polar 
regions. The temperature of the deep water in enclosed por- 
tions of the sea, such as the Mediterranean, in low latitudes, 
never falls to the low temperature of the deep open sea because of 
the raised sea bottom in the straits, which acts as a barrier and 
keeps out the creep of cold water. 

Sounding and Dredging. — The depth of the ocean water and 
the nature of its bottom are studied both for economic and scientific 
reasons. Before submarine cables are laid, suitable routes must 
be determined. 

Soundings of the deep sea are made by means of a weighted 
wire. The weight, called the sounding lead, surrounds a metal tube 
and is attached in such a way that when the tube strikes bottom 
the weight is released and remains on the bottom. The tube 
has a device for bringing up specimens of material found on the 
sea bottom. At intervals along the sounding wire specially de- 
vised minimum thermometers are attached, which record the 
temperature at the various depths reached. It will be seen that 
by a single sounding, not only are depths measured, but tempera- 
tures at different depths and a sample of deep sea deposit are 
obtained. 

By dredging, specimens of deep sea life are obtained. A basket 
of large dimensions and with a flaring opening is dragged along 
the ocean bottom, and various remains and forms of animal life 
brought to the surface. 

The ocean floor has its mountain ranges, its plateaus and its 
plains. There are great volcanic peaks in many places, some of 
which rise higher above the sea bottom than any mountain of 
the land rises above the platform on which it rests. Dolphin 
Ridge is a broad area in mid-Atlantic over which the depth varies 
from 5,000 to 12,000 feet, and is bordered on either side by the 
relatively steep slopes of great troughs in which the water is from 
15,000 to 25,000 feet deep. Chains of islands like Cuba and its 
neighbors are believed to be the peaks of submerged mountain 
ranges. In these major features the ocean floor resembles the land. 



1 88 PHYSIOGRAPHY 

The most striking characteristics of the ocean bottom are the 
smoothness and the absence of the steep slopes so familiar on land. 
Below sea level the slopes of volcanoes and the "abrupt" slope 
at the outer margin of the continental masses are rarely steeper 
than a rise of one foot in twenty. 

There are very few slopes on the ocean floor that would be 
considered' difficult for an automobile to climb, or that are steeper 
than some of the grades on our trunk line railways. 

The smoothness of the ocean floor is due largely to the absence 
of those agents of erosion, which sculpture the land into hills and 
valleys, and also to the accumulation of deposits in depressions. 
Between the shore line and the seaward limit of wave action, 
waves and shore currents are spreading out land sediments, form- 
ing a smooth and nearly level area. Beyond this area deposits of 
several kinds are constantly accumulating, and as the deep water 
here is practically at rest, the sediments settle, filling depressions 
and maintaining a nearly level surface. 

It is interesting to study the way chalk settles from a mixture 
of prepared chalk and water. This mixture is somewhat similar 
to some of the oozes which settle on the ocean floor. We notice 
that the surface of the sediments is more nearly horizontal and 
more regular than that of the bottom of the vessel. This sort of 
action is continually, though slowly, in progress on the ocean floor, 
which is gradually approaching a level surface. 

The Continental Shelf. — Near the borders of the continents the 
sediments brought down by streams, and materials worn from the 
land by the waves, are spread out by the waves and currents, 
forming a gently sloping smooth floor which is called the Conti- 
nental Shelf. The continental shelf is, strictly speaking, a portion 
of the continental mass rather than a portion of the ocean basin. 
It extends seaward to the ioo-fathom line, where the slope, 
becoming steeper, descends to the bottom of the ocean basin 
proper. 

The continental shelf is well developed along the eastern coast of 
North and of South America, and in places is more than ioo miles 
wide. On the western coast it is in most places much narrower. 



GENERAL CHARACTERISTICS OF THE SEA 



189 




Fig. 80. — The Continental Shelf of North America 
After model by Howell. 



The British Islea are on the continental shelf that borders Northern 
Europe. 

There is evidence that much of the area of the continental 
shelf has been above sea level. Several of the valleys of large 
rivers flowing into the Atlantic may be traced 'seaward across 
the continental shelf by valleys or canyons which were cor- 
raded by the river when the continental shelf was a part of the 
dry land. 



190 PHYSIOGRAPHY 

Materials of the Ocean Floor. — The ocean is the great settling 
basin of the world. The rivers are constantly bringing in vast 
quantities of sediment and lesser quantities of dissolved mineral. 
Waves cut into the land and add much to the contribution of the 
streams, and a considerable quantity is added by the winds. 
The solid matter thus received is assorted, transported, and de- 
posited in beds, which may ultimately become sedimentary rocks. 
A large part of the dissolved carbonates is taken up by plants 
and animals, which change it to some such solid form as coral or 
shell, which is eventually added to the deposits of the ocean floor. 

Deposits of the Continental Shelf. — These consist of sand and 
gravel beds, and mud beds. Gravel beds are usually found near 
the mouths of rivers or in localities where the wave action is par- 
ticularly violent. Sand beds sometimes extend many miles from 
the shore. The mud beds are made up of the finest particles and 
are located beyond the sand in the open sea or in the quiet water 
of bays. Pure limestones are formed in clear water beyond the 
mud beds. The deposits on the continental shelf grade into each 
other. 

Deposits of the Deeper Ocean. — Beyond the mud deposits the 
only material derived directly from the land, which accumulates on 
the ocean bed, is the dust from the air, and this is so small in amount 
that it is overshadowed by the organic remains. The waste mate- 
rials of the land extend some distance beyond a depth of 100 
fathoms, but they gradually disappear and are replaced by oozes 
which cover the bottom of the deeper ocean where the depth is less 
than two and one-half or three miles. The oozes consist of mi- 
croscopic shells of animals that live in the surface waters even in 
mid-ocean. When the animals die their shells sink to the bottom, 
forming the soft and grayish deposit, known as ooze. 

Deposits of the Deepest Ocean. — As the depth of the ocean 
increases, the percentage of calcareous matter in the deposits 
decreases, and at a depth of about three miles the deposit is 
chiefly red clay. It seems that at these great depths the minute 
shells and other matter of similar composition which form the 



GENERAL CHARACTERISTICS OF THE SEA 191 

oozes are dissolved before they reach the bottom. The red clay 
consists of the less soluble matter which settles from the air as 
volcanic ash, and dust from meteors, several millions of which enter 
our atmosphere every day. Fragments of pumice and particles 
of meteoric iron occur in the red clay, and the insoluble parts of 
the bodies of animals living on the surface are relatively abundant. 
More than 100 shark teeth and between 30 and 40 ear bones of the 
whale have been brought to the surface at a single haul of the 
dredge. Since there are but two ear bones in a whale, this proves 
that the deposit must accumulate very slowly indeed. 

Life of the Ocean. — x\ll of the great classes of animal life are 
represented in the ocean. Several of the mammals, an order 
whose natural habitat is on land, live in the sea, though it is 
necessary for them to come to the surface to breathe. Among 
them are the whale, porpoise, walrus, seal, and sea lion. No birds 
make their permanent home on the sea, but many aquatic species 
spend much of their time there. Fish of great variety in size and 
form are abundant. Thousands of species of invertebrates of 
nearly every order, from the microscopic protozoan to the gigantic 
squid, are found in great abundance. Among these are the lobster, 
crab, shrimp, oyster, clam, star-fish, and the coral. 

Various species of plants occur almost everywhere along the 
shore. A few of them, like the mangrove and certain grasses, are 
land plants which have adapted themselves to conditions of life 
on the beach; but the majority of the plants are unlike those on 
the land. Some species of seaweed reach great size, larger than 
our tallest trees; but their structure is unlike that of the trees, 
and the weight of the solid matter which they contain is only a 
small fraction of that of our common trees. 

Distribution of Plant and Animal Life. — The distribution of the 
life of the sea is controlled just as is that of the land, largely by 
the climatic conditions of the various parts. The walrus, fur seal, 
and narwhal are found in cold, and the corals only in warm waters. 
The corals and certain allied species are also limited to the regions 
where the water is clear and normally salt; other species, like the 



1 9 2 PHYSIOGRAPHY 

oyster, prefer brackish water and do not require absolute clear- 
ness. 

The depth of water controls the distribution of life as effectively 
as any other varying condition. Light does not penetrate to depths 
much greater than ioo fathoms, and animals and plants requiring 
light must develop above this depth.' 

The temperature of the deep ocean is near the freezing point, hence 
some forms of life are excluded. The pressure in the deeps is 
so great that other forms are excluded. And finally the motion 
of the water is so slight that fixed forms of life, whose food must 
be brought to them, are excluded. 

For these reasons the great depths of the sea are like the desert 
regions of the land in the comparative sparseness of both animal 
and plant life. Such animals as there are have strange forms; 
some of them have eyes, but others are blind. Some of the forms 
probably emit phosphorescent light which enables them to see 
and to be seen. There are no plants in the very deep sea. 

It has been claimed that the life of the sea, as a whole, exceeds 
that of the land, equal areas being compared. It is doubtful, 
however, if life is as abundant in any portion of the sea as it is 
on the more fertile portions of the land. The surface waters every- 
where abound in life. Many species and many individuals of each 
species occur; but both the number of species and the number of 
individuals is greater between the ioo fathom line and the shore 
line than elsewhere. 

Ice in the Sea. — Sea water ordinarily freezes at a temperature between 
26 and 28 F., depending upon the saltness of the water. In the higher 
latitudes ice forms along the shores and also on the deep sea, often to a 
thickness of eight or ten feet. 

The ice formed in winter is usually broken in pieces in the summer. 
These floating pieces, called field ox floe-ice, are often crowded and jammed 
together into an ice-pack, which, because of the lateral pressure, is raised 
considerably above the water. The sea ice may be driven upon the land 
by waves and tides and become twenty feet or more thick by accumula- 
tions of snow. Rock fragments from overhanging cliffs and from the 
imbedding of rocks along the shore, gather upon and in this ice of the 
shore known as an icefoot. In winter the grinding of the ice foot up and 
down the shores smooths and rounds the rocks of these coasts. In the 



GENERAL CHARACTERISTICS OF THE SEA 



193 



summer it breaks up and scatters the rocky material, often long 
distances. 

Glaciers entering the sea from the land in both polar regions break at 
the shore and send off larger masses of ice, known as icebergs. Some 
icebergs are a mile or more in length, and have been known to rise 500 




Fig. 81. — Ice-bound Shores (shaded), and Limits of Drifting Ice in Northern Winter 
(black lines). Dotted lines, Limits of Drifting Ice, Northern Summer 

feet above the water. As ice is nearly as heavy as water, the greater 
part of the floating iceberg is below the surface of the water. The relative 
heights above and below are on the average about 1 to 8. The chief 
work of an iceberg is to transport material in the form of bowlders and 
glacial pebbles, dropping them on the sea bottom in the warmer and more 
open seas. 

QUESTIONS 

1. Where is the great water supply for watering the land? What 
other advantages does the land receive from the sea? 

2. Name the boundaries of the different oceans. Compare the Arctic 
and Antarctic oceans in respect to area. 

3. Calculate roughly the number of cubic miles of water in the At- 
lantic ocean. How does this compare with the volume' of the land mass 
of North America? 

4. What mineral substances and gases are dissolved in sea water? 
How much common salt in a hundred pounds of sea water? What 
causes the sea water to change in density in different localities? 



i 9 4 PHYSIOGRAPHY 

5. Describe the distribution of the surface temperature of the sea in 
different latitudes. Compare the temperature at the surface with that 
at the bottom of the sea, both in the higher and lower latitudes. Ac- 
count for the striking difference in the equatorial regions. 

6. How are temperatures of the deep sea determined? How are 
soundings made? What is the object of dredging? 

7. Compare the ocean floor with that of the land. Account for dif- 
ferences. What is a continental shelf? About how wide are continental 
shelves, how deep is the water upon them, and what purpose do they 
serve? What causes tend to change the area of continental shelves? 

8. What is the character and source of ocean bottom materials? 
How do the deposits differ in different localities? What conditions 
determine the distribution of animal and plant life in the sea. Point 
out specific examples. 

9. Locate the two ice caps of the earth. Under what conditions and 
how is the ice formed? What is the difference between floe ice and ice- 
bergs? What effect does ice in the polar region have upon the land? 



CHAPTER XVI 

MOVEMENTS OF THE SEA 

The most important movements of the ocean are: (i) waves; 
(2) tides; and (3) currents. 

WAVES 

A gentle breeze causes ripples to form on the surface of water 
over which it blows; a strong wind changes these ripples into great 
waves. "During the passage of a wave each particle of water af- 
fected rises and falls and moves forward and backward, describing 
a curved path in a vertical plane. The forward motion of the 




Fig. 82. — Diagram of Wave, Showing Movement of Wave Particles 

Particle 3 is going backward, 7 forward, 5 upward, and 9 and 1 downward. 

From 1 to 5 the particles of water are going backward in the trough, from 5 to q forward on 

the crest, from 3 to 7 upward on the front, from 7 to 9 and from 1 to 3 downward on the 

back. 
What two motions combined has each of the following: 2, 4, 6, and 8? 
How long and how high is this wave ? 
In what direction is the wave form advancing ? 
If this wave should run ashore, would the water at the shore advance first or recede first? 

water is most rapid in the ridge or crest of the wave, and the back- 
ward motion is most rapid in the furrow or trough. The forward 
motion is slightly in excess of the backward motion. Because of 
the excess of forward over backward motion of the water particles, 
when the winds are long continued in the same direction, currents 
are produced which flow in the same direction in which the wind 
blows. On the front of the wave the water rises, and on the back 
of the wave the water falls. As waves move new water enters in 
front and leaves on the back of the wave. 



196 PHYSIOGRAPHY 

Height and Length of Waves. — The horizontal distance from 
the crest of one wave to the crest of the next is the length, and 
the vertical distance between the crest and the bottom of the 
trough is the height of the wave. The height and force of the 
waves depend upon the force of the wind, the length of time the 
wind continues to blow, the depth and breadth of the water, and 
the form and direction of the coast line. 

Ground-Swell. — In the open sea during a gale waves are often 
30 to 40 feet high, and have a length of a thousand feet or more. 
High waves often pass out from an area of storm-winds into a 
region of gentle winds many hundreds of miles away. They be- 
come of less height, but keep their velocity and length. These 
waves that have outrun the storm which started them, and per- 
sist after the storm, are known as the ground-swell. 

Breakers. — When a wave approaches a gently sloping shore 
the wave length is diminished, and the wave height is increased. 
The front of the wave, because of a lack of water, becomes steeper 
than the back; and as the wave continues to move into water of 
less depth the crest curls and falls forward, forming a line of 
breakers. At the line of breakers on a sandy shore a sand bar 
is formed. Rocks or bars near the surface of the water may 
also be located by breakers. Thus breakers are a warning of 
danger. 

Surf and Undertow. — When the waves run into shallow water 
and break near the shore, surf is formed. The water that is then 
thrown forward in the crest of the waves returns as a current along 
the bottom. This backward under-current along the bottom of 
a shallow sea, due to waves and surface currents produced by the 
wind, is called the undertow. When the waves reach the shore 
obliquely, a current along the shore is produced. 

THE WORK OF THE WAVES 

Pounding of the Waves. — Waves are agents of erosion; that is, 
they break and grind the material along the shore and transport 
it varying distances from the shore. 



MOVEMENTS OF THE SEA 



197 



The work of breaking and grinding is done by the fall of the 
breakers upon the shores. In summer, in the Atlantic the average 
blow of breaker is about six hundred pounds on every square 
foot of surface. In winter the force of the breakers may be as 
high as 3,000 pounds per square foot. The impact or pounding 
of the waves on the shores is made effective by the sand, the 
pebbles and such rock fragments as the waves are able to move. 
Driven by the force of the waves, they serve as tools for cutting 
and grinding, and become rounded by acting upon each other. 




Fig. 83.— A Sea Cave 

Weak rocks exposed along the shores are broken down and re- 
moved. The more resistant rocks are loosened by undercutting, 
and because of the joints and seams in the rocks fall as angular 
blocks. These angular blocks in course of time become reduced 
in size and rounded. Large masses of rock, too large at first to be 
moved by the waves, are reduced by smaller fragments driven 
against them until the waves are finally able to use them also as 



iq8 



PHYSIOGRAPHY 



weapons of attack. Thus huge masses of rock are reduced in 
turn to cobbles, pebbles and sand, and finally to the finest mud 
particles, which may be carried away by the undertow. 

Sea-Cliffs and Sea-Caves. — The cutting of the waves at the 
water level may be compared to a horizontal saw. As the waves 
cut into the shore the unsupported material often falls, leaving a 




Fig. 84. — The Action of Waves, Showing Tendency to Follow Joints in the Rocks 

steep face known as a sea-cliff. If the sea-cliff is a wall of rock, 
and the waves continue undercutting at the base, a sea-cave may 
be formed. 

Sea Arches and Chimney Rocks. — If the wearing away of the 
roof continues, the remaining portion may form an arch or bridge. 
Sometimes the waves remove block after block of rock along cer- 
tain joints, so that a column or pillar of rock may be isolated from 
the shore. These are then known as chimney or pulpit rocks. 
The " Old Man of Hoy," on the coast of the Orkney Islands, is 
an example. 

Small irregularities in the shore line develop because of differences 
in the resistance of the rocks, and in their exposure to the attack of 
the waves; but as a rule the action of waves and shore current 



MOVEMENTS OF THE SEA 199 

tends to make the shore line more regular; the projecting head- 
lands are worn away and bay heads are filled. 

In certain places waves wear away the land and deposit the 
material in the sea at a lower level. The rock fragments, pebbles, 
and sand formed at the shore are ground finer and carried away 
by the combined action of waves, undertow, and along-shore cur- 
rents. 

Deposition by Waves, Undertow and Shore Currents. — In other 
localities material is brought in from the sea by the waves and 
deposited on the shore within the zone of wave action, and forms 
the beach. When the material carried out by the undertow 
meets that brought in by the waves, an accumulation begins at 
the place of meeting. A low ridge called a barrier is formed, and 
its position is shown by the line of breakers. Such barriers are 
often built up to and above the surface of the water, making a 
sand reef. 

The free end of a beach or a barrier is called a spit. The de- 
posits along the shore depend largely upon the shore currents. 
The growth westward of Rockaway Beach, on the southern shore 
of Long Island, is due partly to along-shore currents in that 
direction. 

The growth of shore deposits tends to fill up bay entrances and 
interfere with navigation. At the entrance to New York harbor 
dredging is necessary in order to deepen the channels through 
which the largest boats pass. 

TIDES 

Tides Defined. — Along the shores of the ocean and its gulfs and 
bays the water rises slowly for about 6 hours and 13 minutes, and 
then falls slowly for about the same time, making on an average 
12 hours and 26 minutes from high water to next high water, or 
from low water to next low water. This periodic rise and fall of 
the level of the sea twice in every 24 hours and 52 minutes constitutes 
the tides. 

This makes the hour of high water at any particular place vary 
from day to day. If it is high water at the ocean shore this after- 



200 PHYSIOGRAPHY 

noon at 4 o'clock, the next high water will occur again 26 minutes 
past 4 to-morrow morning, and high water again 52 minutes past 
4 to-morrow afternoon, and so on. 

Variation in Tidal Range. — The amount of rise and fall is 
greater along most continental coasts than in mid-ocean, and 
greatest in bays with broad openings to the sea and narrow toward 
their heads. The tidal range at Key West, Florida, is usually 
not more than two feet, while in the Bay of Fundy it is often 
more than 50 feet. 

The amount of the rise and fall of the sea at any particular place 
also varies. The tidal range may increase from day to day for 
about a week and then decrease for the same period, making a 
maximum and minimum range twice a month. At Governor's 
Island in New York Harbor the tidal range may be as small as 
3.4 feet, and as great as 5.3 feet during a single week. 

Flood and Ebb Tides. — The change of level of the sea is accom- 
panied by tidal currents called the running of the tides. When 
the tide is running from the open ocean into bays, it is flood or 
incoming tide; and when the tide runs to the open ocean again, 
it is the ebb or outgoing tide. During the few minutes when the 
flood tide changes to ebb tide or ebb to flood slack water occurs. 

Tidal Races. — When the tidal currents pass through a strait, 
such as a narrow inlet into a bay or between an island and the 
mainland, the currents often run many miles an hour. Such 
currents are called tidal races, and are often so strong as to inter- 
fere with navigation. The tidal currents " race " through Hell 
Gate, the narrow passage from the East River into Long Island 
Sound, at the rate of five or six miles an hour. 

Tides in Rivers.— The tidal wave often runs up rivers to a point 
many feet above sea level. The tide runs 150 miles up the Hud- 
son River to Troy, five feet above sea level, where the tidal range 
is more than two feet. The tide is felt 70 miles up the St. John 
River in New Brunswick, where the elevation is fourteen feet above 
sea level; and at Montreal, 280 miles up the St. Lawrence River. 



MOVEMENTS OF THE SEA 201 

The action of tidal currents in narrow rivers is very different 
from the action of tidal currents on open seacoasts. In rivers, 
when the water stands above the average level, the tidal current 
flows up-stream along with the tidal wave, and when the water 
stands below the average level the tidal current flows down- 
stream, opposite to the direction of the tidal wave. Since the rate 
of flow depends upon the difference in level, the flow is most rapid 
at high and low water instead of being slack water at these times, 
as on open coasts. Hence the tidal current flows up-stream for 
some time after high water has passed and the water level is 
falling; and the tidal current flows down-stream for some time 
after low water is reached and the water level is rising. In broad, 
deep mouths of rivers, slack water does not occur at high and 
low water as on open coasts, nor at average level as in narrow shal- 
low rivers, but at some intermediate level. 

Tidal Bore. — In the estuaries of many rivers broad flats of mud 
or sand are nearly exposed at low water. The tidal wave when 
entering these rivers often rises so rapidly that it assumes the 
form of a wall of water. Such a wave is called a bore. Tidal 
bores occur in some of the rivers of China, where in one case the 
bore travels up the river at every high tide, often reaching a height 
of twelve feet. After the bore has passed, an after-rush often 
carries the water up several feet higher. 

Bores have been observed on the Severn in England, on the 
Seine in France, on the Amazon in South America, and on a few 
other rivers of the world. 

Causes of the Tides. — Since Newton announced the law of uni- 
versal gravitation it has been generally recognized that the tides 
result from the attraction of the sun and moon. The tide-pro- 
ducing forces of sun and moon can be computed with reasonable 
certainty, but because of the modified effects due to local condi- 
tions an agreement between theoretical and the actually observed 
tides is not easily secured. Although the moon's mass is only a 
small fraction of the sun's mass, the moon's nearness to the earth 
makes it, rather than the sun, the principal cause of the tides. 



202 PHYSIOGRAPHY 

First Law of Motion. — A body in motion will move in a straight 
line unless deflected from its straight path by some external force. 
This law of motion may be illustrated by whirling a stone around 
the hand by means of a string. The natural tendency of the stone, 
at each instant, is to move in a straight path. It is deflected and 
moves in a curved path because of a pull or force, called centrip- 
etal force, exerted by the string acting inward upon the stone. 
The stone resists being pulled inward and so tends to move out- 
ward, and exerts a pull or force upon the hand called centrifugal 
force. The string being under tension when the stone is whirled, 
is subject to equal and opposite forces, one acting toward 
(centripetal), and the other away from (centrifugal), the center 
of revolution. 

Balance between Centripetal and Centrifugal Forces. — The 

revolution of the moon about the earth is illustrated by this simple ex- 
periment. The invisible force called the gravitation which acts between 




Fig. 8s 

the moon and the earth replaces the centripetal force exerted by the 
string that holds the stone to the hand. The moon whirls about the 
earth with sufficient velocity and at such a distance that her resistance 
to curved motion, or centrifugal force, just equals and balances the at- 
traction between the earth and moon. 

Center of Gravity. — The moon does not revolve about the center 
of the earth, but about a point 3,000 miles from the center, or 1,000 
miles below the surface. This is because the earth is eighty times as 
heavy as the moon and the centers of the two bodies are 240,000 miles 
apart. 



MOVEMENTS OF THE SEA 203 

This may be easily illustrated by balancing two balls, one eighty times 
the weight of the other and connected by a slender rod. The place where 
they balance, called the common center of gravity, will be one-eightieth 
of the distance from the center of the larger ball to the center of the 
smaller. 

Revolution About Common Center of Gravity. — The common cen- 
ter of gravity of the earth and moon is at C. The big and little balls cor- 
respond to the earth and the moon, and the stress in the rod represents the 



Common renter of gravity 
of earth and moon 






M 



Fig. 86 

attraction that holds the earth and moon together. Both the earth and 
the moon revolve about this common center of gravity, C, in about 28 
days, and in so doing the earth's center describes a circle with a radius 
of 3,000 miles. 

The daily rotation of the earth, which is not now being considered, 
must not be confused with the revolution of the earth, without angular 
turning, about a point 1,000 miles below the earth's surface. Only the 
earth-moon revolution about C without rotation of either body is here 
considered. When a body revolves about another without rotation, a 
given side always faces the same direction in space. 

Revolution without Rotation. — It may be stated as a general prop- 
osition that whenever an object revolves without rotation, every particle 
of the object describes a path the size and shape of that described by a 
particle at the center of the object. The motion of the different particles 
of a connecting rod attached to the driving wheels of a locomotive 
illustrates this action. 

All parts of the earth then must be subject to equal and parallel 
centrifugal forces, due to the monthly revolution of the earth and moon 
about their common center of gravity. These forces act in a direction 
away from the moon. The total of centrifugal forces acting on the earth 
is just balanced by the total centripetal force due to the moon's attrac- 
tion, although it is evident that the two opposite forces acting on any 
single particle are only equal at the center of the earth. 



204 PHYSIOGRAPHY 

Unequal Attraction of the Moon in Different Parts of the Earth. — 

The moon's attraction for the earth is always toward the moon, but 
is not equally distributed, for the attraction on the side of the earth 
nearest the moon is stronger than at the center, and on the side of the 
earth farthest from the moon weaker than at the center. 

Resultant of Two Opposite Forces. — In the figure, A B C D, repre- 
senting the equator of the earth, A is a particle farthest from the moon; 
C a particle nearest to the moon, and E a particle at the center of the 
earth. The arrows of equal length, extending to the left away from the 
moon, represent the equal centrifugal forces; and the arrows of unequal 




Fig. 87. — Opposite Forces 



lengths, extending to the right toward the moon, represent the unequal 
value of the moon's attraction at these points. 

When two forces act in opposite directions at the same point, the 
effectiveness or resultant of the two forces is found in a force equal to 
the difference between the two and acting in the direction of the greater 
force. 

At C the moon's attraction is greater than the centrifugal force at that 
point, so that the tide-producing force, which is the difference or resultant 
between these forces, acts toward the moon and causes the water on the 
side of the earth toward the moon to bulge out toward the moon. 

At A the moon's attraction is less than the centrifugal force, and the 
tide-producing force consequently acts away from the moon, and causes 
the water on the opposite side of the earth to bulge out away from the 
moon. 

At E the moon's attraction and the centrifugal force are equal and 
opposite. If they were not, the earth and moon would either approach 
or recede from each other. 

These two bulges of the ocean are the two high tides, and midway 
between them is the low tide zone. 

The magnitude and direction of the resultant or tide-producing forces 
acting at different points on the earth's equator are shown in Figure 87. 



MOVEMENTS OF THE SEA 205 

Effect of Rotation of the Earth. — The daily rotation of the earth 
from west to east constantly carries the high and low tide west- 
ward around the earth, and brings places alternately to high tide 
and low tide positions. 

The tidal movements are interfered with by the continents 
which tend to stop or change the direction of the tidal wave. 
The tidal wave travels faster in the deep ocean than in the shallow 
water near the continents. The tidal waves are also interfered 
with by the strong winds and changes of atmospheric pressure. 
Their advance in different parts of the ocean becomes so irregular 
that they often interfere with one another. This explains in some 
measure why the actual local tides in so many places fail to agree 
with the general theory. 

The Establishment of the Port. — The rotation of the earth tends 
to carry the tidal waves forward in the direction of the rotation. 
The moon tends to hold the tidal waves back. The result is that 
the tides are said to lag. The interval of time between the pas- 
sage of the moon across the meridian and the next high tide, 
mariners call " the establishment of the port." The establish- 
ments of different ports have various values. The port of New 
York has a value of 8 hours and 13 minutes. 

Cause of Solar Tides. — The explanation of solar tides is analo- 
gous to that of lunar tides. Since the cause of lunar tides is the 
difference between the moon's attraction and centrifugal force 
in different parts of the earth, in like manner solar tides are 
due to the difference between the sun's attraction and cen- 
trifugal force in different parts of the earth, caused by the earth 
moving about the common center of gravity of the earth and 
the sun. 

Effect of Solar upon Lunar Tides. — The intensity of the tide- 
producing force due to the sun is about half of that due to the 
moon. Since the lunar tides are stronger than the solar tides, 
the solar tides may be said to modify them, that is, to strengthen 
the tides when sun and moon act together, and to weaken them 
when they oppose each other. 



206 PHYSIOGRAPHY 

Twice a month, at times of new and full moon, the lunar and 
solar tides fall together, producing a higher tide than usual. This 
condition of greatest range is called spring tide. At first and last 
quarters of the moon the solar high tide falls at lunar low tide, 
and solar low tide falls at lunar high tide. The effect of this is 
to lessen the tidal range, that is, the high tides are not so high 
and the low tides are not so low as usual. This condition of least 
range is called neap tide. 

The relative ranges of spring and neap tides may be shown 
graphically by the construction of tide curves for any station. 
The data for these tide curves may be found in tide tables pub- 
lished by the Government. 

The tides in any latitude vary with the changing angular dis- 
tance of the moon and sun north or south of the equator, as well 
as with their changing distances from the earth. 

Inequality of Tides. — The two successive high tides of a given 
place are usually of unequal height. They are of equal height only 
when the moon is over the equator, and as this occurs on only two 
days of the month two weeks apart, the two successive high tides 
are usually unequal. The maximum inequality of successive high 
tides occurs when the moon is farthest north or south of the equa- 
tor. This variation at some places amounts to several feet. 

Maximum Yearly Tide. — The conditions that favor the greatest 
tidal range in any particular harbor are: (i) new or full moon; (2) 
moon and sun nearest to the earth; (3) moon and sun's zenith 
distances approximating the latitude of the place affected; (4) wind 
direction favorable to direction or tidal movement. 

Effect of Tides. — The erosion caused by tidal currents is known 
as tidal scour. The tidal scour of the flow and ebb of the tide 
maintains inlets in barrier reefs along many shores. An example 
of this may be seen in the sand reefs along the shore of New 
Jersey. Tidal scour also often maintains deep waterways in some 
bays to the advantage of navigation; whereas at the entrance to 
other bays the tidal currents tend to fill, making the water shal- 
low, and because of shifting of deposits are dangerous to naviga- 



MOVEMENTS OF THE SEA 



207 




208 PHYSIOGRAPHY 

tion. Strong tides hinder the formation of beaches across the 
entrance of some bays. 

The tidal currents cause a circulation of water in bays and har- 
bors which prevents stagnation and helps to remove the sewage that 
is drained into them near cities. This circulation of water aids or 
hinders boats, according to their direction, and sometimes drifts 
vessels out of their course and subjects them to danger of rocks and 
shoals, especially in times of dense fogs. 

Tidal currents transport material along shore from more ex- 
posed positions, such as headlands, to the less exposed position 
at the heads of bays. This filling of bay heads tends to straighten 
the shore line. 

CURRENTS 

Every continent is washed by ocean currents, and every ocean 
has its distinct circulation. Currents from equatorial regions 
carry warm water into polar regions, and other currents carry the 
cold polar waters into lower latitudes. 

While each ocean has its separate circulation, yet the separate 
schemes of circulation fit into the general scheme as cog-wheels 
in a vast machine. 

The Pacific Ocean, which for most purposes is considered as one 
ocean, is by reason of its circulation divided into two distinct 
parts, the North Pacific and the South Pacific. The Atlantic and 
Indian oceans lying, like the Pacific, on both sides of the equator, 
are also divided into northern and southern oceans by reason of 
their distinct circulation. 

Systematic Movement. — Ocean currents, like air currents, obey 
Ferrel's Law, in that they turn to the right of a straight course in 
the northern hemisphere, and to the left in the southern. This 
results in a distinct eastward drift about the margin of the south 
polar ocean, and a less distinct eastward movement about the 
Arctic Ocean. In other oceans the northern divisions have a 
clockwise circulation, whereas the southern divisions have their 
circulation counter-clockwise. 

The movement of the waters in all oceans is chiefly about the 




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MOVEMENTS OF THE SEA 209 

margins, leaving the great central areas undisturbed. In these 
areas of quiet water seaweed and other floating matter accumu- 
lates, thus producing what are known as Sargasso Seas. These 
seas are avoided by masters of sailing vessels, who find it difficult 
to get out of these drift-covered waters when driven into them 
by storms. Columbus thought, when he came to the Sargasso 
Sea in the Atlantic, that he had come upon land. 

Causes of Currents. — Anything that produces a disturb- 
ance of the level of the ocean surface will, at that place, cause 
currents. 

In the trade wind belts the hot dry winds cause a slight lower- 
ing of the surface by evaporation, and there is a natural tendency 
for the waters to flow in both from the north and the south. 

In the doldrum belt, the excessive rainfall slightly raises the 
surface of the sea, and the water flows out to north and to south. 

Great storms, as at Galveston, pile the water up against the 
land, often with great destruction, and the return of the sea to 
its normal level produces local currents. 

Differences of temperature, while effective in producing vertical 
currents when the heavier water is at the top, can produce hori- 
zontal movement only when by reason of these differences of 
temperature the surface of the sea is raised or lowered. This may 
cause a slight raising of the surface at the equator or a slight 
lowering of the surface at the poles. 

While these various causes may produce local currents, they 
do not account for the systematic circulation of the oceans. 
There remains to be considered the all-sufficient cause, the winds. 

Origin of Ocean Currents in the Trades. — All winds, however 
fitful, brush the surface water along with them. If they constantly 
vary in direction, no systematic or continuous currents can result. 
When the same direction is held for several days,' a distinct drift 
with the wind is observed. 

Continued east winds over Lake Erie have at times so heaped 
up the water toward the west end that Niagara Falls have prac- 
tically run dry. We are told, too, that strong east winds some- 



2IO PHYSIOGRAPHY 

times drive the waters back from one of the northern arms of the 
Red Sea, and make it possible to cross this basin " dry-shod." 

It is only in the trade wind belts that we find winds blowing 
continuously from the same direction; and we are disposed to 
look upon these belts as the birthplace of ocean currents. Here 
the direction of the ocean currents agrees with that of the trades, 
and neither difference of density nor difference of temperature 
can have any part in producing this westward movement of the 
ocean waters. These are the north and south equatorial currents. 

Poleward Currents. — The equatorial currents are barred in 
their westward movement by islands and continents across their 
paths. They are thus forced to turn poleward along the western 
shores of the oceans. Whether they turn northward or southward 
is determined by the outline of the coast. 

While the currents are moving along and near the equator, the 
earth's rotation has but slight deflecting influence; and it is prob- 
able that, if not interrupted by land barriers, the equatorial cur- 
rents would continue their westward course around the earth. 

As soon, however, as they begin to flow into other latitudes, the 
rotation of the earth is effective in turning them from a straight 
course to the right in the northern hemisphere and the left in the 
southern. 

These poleward currents are warm currents, and carry the warm 
water from the equatorial regions into colder latitudes. At the 
same time they spread out, lose their velocity, and are then known 
as drifts, which move to the margins of the polar oceans, then 
eastward to the eastern shore of the ocean in which they have 
their origin. 

Equatorward Currents. — By continued deflection, these east- 
ward moving currents, now cooled from loitering in high latitudes, 
are turned toward the equator along the western coasts of the 
continents. Returning thus to the trade wind belts, in which 
they assume their westward direction, the circulation about the 
ocean is complete. 

The equatorward currents are cold or cool currents, and bring 



MOVEMENTS OF THE SEA 211 

lower temperatures toward or even to the equator, causing the 
eastern sides of equatorial oceans to be cooler than the western 
sides. 

The movement of currents about the Arctic Ocean is less syste- 
matic than about the Antarctic, because of the numerous islands 
in the north that interrupt. Branches from the circumpolar move- 
ment in the north are sent off southward into the Pacific and the 
Atlantic. These cold currents deflected to the right, follow closely 
the eastern coasts of Asia and North America, until they sink 
beneath the warm currents between the parallels of 40 and 
5o° N. 

Creep. — Their further journeying toward the equator is known 
as creep. In this way the cold polar waters are carried even to 
the equator, and the low temperatures of deep equatorial seas 
are accounted for. We cannot observe the creep, but as more 
surface water is carried into polar regions than returns as surface 
currents, the excess must be equalized by under-surface return 
currents. 

Monsoon Currents. — If any doubt existed as to the sufficiency 
of the winds to produce ocean currents, that doubt would be 
removed by a study of those currents which change their direc- 
tion with the change of direction of the monsoons. 

While there are monsoons at the Horse Latitudes, the winds 
there are of neither sufficient strength nor constancy to be effective 
in producing ocean currents. It is in the monsoon belt over 
which the heat equator migrates that we find conditions favorable 
for the production of ocean currents. 

About the northern Indian Ocean, when the southwest monsoon 
blows, the water is set drifting in a clockwise direction. As these 
winds weaken, this drift slackens; and soon after the northeast 
monsoon begins, the direction of the drift is reversed. It con- 
tinues as a counter-clockwise circulation while the northeast 
monsoon continues, changing again to the clockwise direction with 
the return of the southwest monsoon. These changes of direction 
of the ocean currents can be accounted for only by the reversal 
of the winds. 



212 PHYSIOGRAPHY 

In the Pacific Ocean, where the heat equator lies prevailingly 
north of the terrestrial equator, the southeast trades, changed to 
southwest winds north of the equator, set up an ocean drift to 
eastward. This is the Equatorial Counter Current. It is fairly 
distinct throughout the year, though better developed during the 
northern summer. Its explanation is the same as that of the 
clockwise movement about the northern Indian Ocean during the 
southwest monsoon. 

Because of the narrowness of the Atlantic Ocean at the equator, 
the counter-current is not so well developed as in the Pacific. 

Currents and Navigation. — Sailing vessels lay their courses to 
suit the winds and ocean currents; and even steamships do not 
scorn to take advantage of the great ocean circulation. 

Sailing vessels from New York to English ports take advantage 
of the northeast Atlantic drift; on their return they use the 
trades. Those bound from New York to Rio Janeiro must lay 
their courses far to eastward of the eastern cape of South America, 
lest the equatorial currents carry them northward again while in 
the doldrums, where winds are apt to fail. 

Ships sailing from Atlantic ports for Australia sail eastward 
around the Cape of Good Hope, to take advantage of the Antarc- 
tic drift ; while those returning also sail eastward past Cape Horn, 
to have the advantage of the same drift. 

Vessels bound from Honolulu to San Francisco sail northward 
beyond the trades and equatorial current, then east; returning, 
they take a more southerly route. 

Currents and Life. — The distribution of many marine forms is 
determined by the temperature of the water, which in turn is in 
part determined by ocean currents. Corals serve well to illustrate. 
The waters about the Galapagos Islands are too cold for corals, 
although these islands are situated upon the equator. The cold 
Peruvian current makes these waters cold. Contrasted with these 
are the Bermudas, in latitude about 35 ° N., which are largely 
composed of coral rock and bordered by coral reefs. The warm 
waters are brought to these islands by the Gulf Stream. 



MOVEMENTS OF THE SEA 213 

The seeds of many plants are distributed by means of ocean 
currents; and insects and the smaller animals are carried upon 
drifting materials in these currents. 

Currents and Climate.— The direct climatic influence of ocean 
currents is confined to the ocean and immediately bordering 
lands. Indirectly their influence may be felt hundreds of miles 
inland. This is markedly true of lands lying to leeward of cur- 
rents that are abnormally cold or warm. 

The North Atlantic Drift, the continuation of the Gulf Stream, 
is perhaps the most pronounced and far-reaching of all ocean 
currents in its climatic influence. The winds from over this broad 
sheet of warm water not only bring abundant rainfall to the 
British Isles and Norway, but so temper the cold of these high 
latitudes as to make them comparable in temperature to our 
own eastern coasts, twenty degrees farther south. 

The North Pacific Drift, the continuation of the Japan Current, 
tempers the climate of Alaska and British Columbia in like 
fashion. 

These great drifts, in both oceans, continue or send branches 
southward along the western coasts of the continent; and when 
they reach the latitude of northern Mexico and Africa, their 
effect is to temper the heat of these coasts. 

The cold currents that follow closely the eastern coasts of 
North America and Asia, being to leeward of those continents, 
do not affect the climate so far inland. However, the bleakness 
of Labrador and Kamchatka is in some degree traceable to these 
currents. 

In the southern hemisphere the western coasts are cooled and 
the eastern coasts warmed by the ocean currents; but their influ- 
ence is less pronounced than in the northern hemisphere. 

Currents and Harbors. — The harbor of Hammerfest, at the 
north of Norway and well within the Arctic Circle, is about as 
free from ice as that of Boston, 30 farther south. In the one 
case we see the effect of the warm North Atlantic Drift; in the 
other, of the cold Labrador Current. 



214 PHYSIOGRAPHY 

In the Pacific Ocean the barrier of the Aleutian Islands, to- 
gether with the narrowness of Bering Strait, prevents the North 
Pacific Drift from entering the Arctic Ocean. As a result, the bays 
on the north coast of Alaska, in the same latitude as Hammerfest, 
are practically closed by ice throughout the year. 

The Russian- Japanese War had for one of its objects the secur- 
ing for Russia of the open harbor of Port Arthur. The harbor of 
Vladivostock, Russia's chief port on the Pacific, in about the 
latitude of New York, is for a long time every year closed by 
ice, owing to the cold current coming down through Bering Strait. 

The Gulf Stream. — This greatest and most important of all 
ocean currents derives its name from the Gulf of Mexico, from 
which it issues. It is in fact a continuation of the combined 
equatorial currents. 

The North Equatorial Current in the Atlantic is turned by the 
land masses in its path wholly into the northern division of this 
ocean. Much of its waters pass among the islands of the West 
Indian group, while the remainder passes to the eastward. 

The eastern cape of South America is so situated that it divides 
the South Equatorial Current in two, part of it turning southwest 
along the coast of Brazil as the Brazilian Current, while the other 
part enters the Gulf of Mexico between the West Indies and the 
mainland of South America. This water issues through the Strait 
of Florida as the Gulf Stream. It is truly a stream, flowing between 
banks of water. It is there deep and narrow, scouring the bottom 
of the strait, and flows with a velocity greater than that of the 
lower Mississippi River. 

Joined by the waters that come through the West Indian group 
of islands, and that which passes outside, the Gulf Stream is 
greatly increased in volume. It passes parallel to and near enough 
to the Carolina coasts to send off return eddies, which build the 
Carolina capes. Spreading and decreasing in velocity, the Gulf 
Stream becomes the North Atlantic Drift. 

The frequent and dense fogs off Newfoundland are produced by 
warm winds from the North Atlantic Drift, blowing over the cold 



MOVEMENTS OF THE SEA 215 

Labrador Current. The line of meeting of the cold and warm 
waters is known as the cold wall. 

QUESTIONS 

1. Tides resemble waves in many respects. High and low tides 
correspond to what parts of the wind wave? Tidal currents correspond 
to what phenomenon of the wind wave? The change in tidal range, 
the velocity and form of the tidal wave as it advances in shallow water 
on the continental shelf and into bays may be compared to what changes 
in the wind wave as it moves toward the shore? Compare the height 
and length of wind waves with that of tidal waves. 

2. Is sea-sickness more likely to occur on large or small boats? Why? 
What is the difference between surf and a breaker? What work is done 
by breakers and the undertow? Why are breakers a warning of danger? 

3. Explain how the waves act as a horizontal saw cutting into the 
land. What are some of the shore features resulting from wave action? 
What effect has these features upon the value of harbors and shore 
property? 

4. How would a thoughtful person living at the shore for any 
length of time naturally connect the cause of the rise and fall of the sea 
with the moon? 

5. Explain how navigation is affected by (a) Tidal range; (b) Flood 
tide; (c) Ebb tide; (d) Tidal races; (e) Tidal bores. How do you 
think the state of the tide affects fishing? 

6. How can one moon cause two daily tides, or in other words, what 
is the cause of a high tide on the side of the earth opposite to that of 
the moon? 

7. Which has a lower low water, a spring or a neap tide? Explain. 
How often does the moon cross the equator? What effect has this on 
the height of the two daily tides? 

8. What effect has tidal scour upon waterways, inlets, and tidal 
streams? What is the general effect of tides upon the water and shores 
in and about bays and harbors? 

9. What is an ocean current? How fast do they flow? How deep 
are they? Describe a particular current in detail. 

10. What is meant by the cog-wheel scheme of circulation? What is 
a Sargossa Sea? 

n. What is the general cause of ocean currents? Point out definite 
evidence. Name and locate several ocean currents. What is a "creep"? 

12. What is the effect of ocean currents upon climate? Point out 
specific examples. What is the effect of ocean currents upon navigation? 
Point out specific examples. 



PART IV 

THE LAND 



CHAPTER XVII 
THE MANTLE ROCK 

Structure of the Solid Earth. — Everyone is familiar with the fact 
that solid rock appears on the surface of the land in but few places, 
and that this surface nearly everywhere consists of loose or uncon- 
solidated earthy matter. This is the mantle rock. In some places 
it reaches a thickness of several hundred feet, but as a rule, the full 
thickness is revealed in stream valleys, and one can find such sec- 
tions as that shown in Fig. 89 in nearly all ravines. 

The solid rock which underlies the mantle rock is called the bed 
rock. In the ordinary sense the term rock does not include loose, 
fragmental deposits, but natural formations of the same origin 
show all degrees of consolidation from that of sand to the hardest 
sandstone. We therefore define rock as a natural deposit of earthy 
matter, whether consolidated or not. 

Economic Importance of the Mantle Rock. — The mantle rock 
is of the greatest economic importance. Without it the surface 
of the land would be solid rock, and agriculture would be impossible. 

All the mantle rock, except the layers of pure clay, permits water 
to pass readily through it, thus acting as a distributer of water. A 
portion of the rainfall sinks into it, and through the action of grav- 
ity is slowly distributed to all parts below the water table. Above 
the water table, water is diffused by capillary action. 

The mantle rock acts as a great reservoir which receives and 
temporarily stores a large portion of the rainfall, thus tending 
to prevent floods which would otherwise occur after every heavy 
rainstorm. The quantity thus conserved is much greater than 
that conserved by the forests, important as is this latter amount. 
The water in this reservoir supplies wells and springs, keeps 
plants alive in dry weather, and much of it gradually makes its 



220 



PHYSIOGRAPHY 



way into the streams, furnishing a supply of water even in dry 
seasons, thus making the larger streams permanent and fairly uni- 
form in size. A large portion of the water supply of Brooklyn, 
N. Y., is obtained from wells that do not reach the bed rock and 
from which several million gallons per day are pumped. 

The mantle rock is a natural filter. Rain washes the air, beating 
down dust particles and removing disease germs. On the surface 




Fig. 89. — Natural Section Showing Mantle Rock and Bed Rock 
Lockport, New York. Geological Survey of New York. 

of the earth it becomes muddy and is contaminated in many ways, 
making the surface water unsafe for household use. The water of 
wells and springs is clear because the mantle rock has filtered it, 
and if wells are not too shallow the water is generally pure and 
safe to use. 

The mantle rock is a great storehouse of plant food. As it is a 
poorer conductor of heat than the solid rock it acts as a blanket, 
diminishing the earth's loss of heat by radiation. 

Origin of the Mantle Rock. — The mantle rock consists of frag- 
ments of bed rock in various stages of disintegration and decay, 
that have been loosened and changed through the action of a num- 
ber of natural agents which accomplish the result in different ways. 

The quiet action of the atmosphere, with its moisture and its 
changes in temperature, slowly disintegrates solid rock, and in this 
manner has formed much of the mantle rock and is of great im- 



THE MANTLE ROCK 



221 



portance. This process is known as weathering. Glaciers and 
running water wear away the surface of the rock over which they 
move and add the loosened particles to the mantle rock. 

An appreciable addition to the mantle rock results from the 
action of wind-blown sand and the waves on solid rock. Figs. 




Fig. qo — Ovoidal Block of Granite 

Produced by weathering. Redstone Quarry, Westerly, R. I. 

From U. S. Geological Survey. 

94 and 97 show rock that has been much worn by wind-blown 
sand and Fig. 83 by wave action. 

The most important source of mantle rock is weathering. 

Weathering. — Every boy has learned that the stones found in 
the fields differ greatly in hardness and strength. Sometimes one 
finds a stone that will crumble in one's hands or that will scale off 
on the outside and is well preserved and hard in the center. Such 
specimens illustrate weathering. 

The difference in the appearance and the solidity of freshly 
quarried rock, and that of the same rock which has been exposed 
long to the action of the elements, is due to weathering. The 



222 



PHYSIOGRAPHY 



stones of many buildings less than a quarter of a century old show 
the effect of weathering, and some of the stones that are used 
extensively for building in the United States, weather to such an 




Fig. 91. — Granite Broken by Internal Stress and Afterward Weathered 
The rounded forms and apparent stratification were caused by rapid weathering along the 

lines of fracture. 

extent in a few years that it is necessary to protect them in some 
manner to prevent their entire destruction. 

Weathering is the term applied to the various natural processes of 
softening and disintegrating the surface layers of rock exposed to the 
atmosphere. 

Chemical Weathering. — Certain agents of weathering attack 
rock in practically the same way that articles made of iron are 
attacked when they rust. These agents produce chemical changes 
in the rock and the products of their action are new substances 



THE MANTLE ROCK 



223 




Fie. 92. — Hoodoo Basin. Ahoaraka Range. Yellowstone Park 
Showing fantastic forms carved from igneous rock by rain and weathering. 



entirely unlike the original, just as iron rust is unlike the iron from 
which it was formed. These are the chemical agents of weathering. 
The most important chemical agents concerned in weathering are 
oxygen, carbon dioxide, and water. 



224 PHYSIOGRAPHY 

Oxygen. — This is the most active of the elements in the air. 
In the presence of moisture it not only combines with iron and a 
number of other metals but it also attacks many compounds found 
in the rocks, uniting with them and forming new compounds. A 
ledge of rock is often easily crumbled and of a brownish or yel- 
lowish color on the outside, and firmer within. Such changes are 
due to the action of oxygen, and the changed substance is said to 
be oxidized. 

Carbon Dioxide. — This is another constituent of the air which 
corrodes rock. It is most active when dissolved in water. The 
igneous rocks are largely composed of complex minerals and are 
decomposed by water containing carbon dioxide. When the con- 
stituent minerals contain calcium, one of the products of this 
action is calcium carbonate. Being soluble the calcium carbonate 
thus formed is carried to the sea by streams, where much of it re- 
appears in solid form as limestone. 

Water. — Water often combines with some of the constituents of rocks, 
with an increase in volume which causes the remainder of the rock to 
crumble. Certain micas illustrate this action, and this probably accounts 
for the rapid weathering of micaceous sandstones. 

Other Chemicals. — Nitric acid formed in the air by lightning, certain 
sulphurous gases erupted by volcanoes, and acids formed by decaying 
vegetation also produce chemical changes in rocks which result in their 
disintegration. 

Mechanical Weathering. — Certain agents abrade rocks in the 
same way that a file wears away iron. This is a mechanical process 
and the products remain the same material as the original sub- 
stance, just as iron filings are the same material as the piece of iron 
from which they were separated. Other agents disintegrate rocks 
by blows like those of particles of flying sand. All of the agents 
which disintegrate without changing the identity of the material 
are mechanical agents. 

Changes in Temperature. — When stone is heated or cooled it 
expands or contracts. If the heating or cooling is slow enough to 
change the temperature uniformly throughout the mass the effect 



THE MANTLE ROCK 225 

is slight. If, however, the rock is unequally heated or cooled it 
produces the same sort of stress in the rock that is produced in a 
glass jar when hot fruit is poured into a cold jar. This stress 
caused by unequal expansion of different parts frequently breaks 
the rock just as it does the fruit jar. Both glass and rock are poor 
conductors of heat, and, therefore, when the surface of either sub- 
stance is heated the temperature of the surface rises more rapidly 
than that of the interior, thus establishing the condition of stress 
which tends to disrupt the substance. Every ledge of rock upon 
which the sun shines is subjected to this action to a greater or less 
degree, and when the daily range of temperature of the rocks is 
large, as it is in high altitudes, expansion and contraction is 
sometimes the most effective agent concerned in local weathering. 

When a layer of rock has been uncovered so as to receive the 
sun's rays, as at the bottom of a stone quarry, the resulting rise 
in temperature expands the rock, producing tremendous lateral 
pressure which sometimes causes the rock to buckle and break. 
This pressure is increased in the daytime and diminished at night. 
These daily fluctuations in stress are effective in weakening the 
cohesion of the rock, thus assisting in weathering it, and the vary- 
ing lateral pressure may materially aid in displacing the adjoining 
rock. 

In New York City a cement sidewalk 700 feet long and 15 feet 
wide was completed in February. One warm day the following 
June the lateral pressure due to the high temperature caused the 
sidewalk to buckle in three places, raising three miniature moun- 
tain ranges nearly a foot high across the walk. The stone was 
much broken at these places. It was repaired in July and has not 
since repeated the phenomenon. Why? 

When the Chicago and Northwestern Railroad was in process of 
construction a portion of its line along the shore of Devil's Lake, 
Wisconsin, passed over a large mass of very hard rock, quartz- 
ite, occupying a narrow space between a nearly vertical cliff of the 
same substance and the shore. After expending large sums of 
money experimenting with various kinds of drills, including the 
diamond drill, in an effort to remove the rock by blasting, they 



226 



HYSIOGRAPHY 




Fig. 93. — -Top of Pike's Peak, Showing Rock Broken by Freezing and Thawing 

were about to abandon the work when someone suggested that 
wood fires be built upon the rock and that when the rock was well 
heated a stream of cold water be thrown upon it. The plan was a 




j* 1£Z9»' 



Fig. 94. — Effects of Wind-blown Sand (Arizona) 
By permission of Oliver Lippincott. 



THE MANTLE ROCK 



227 



success and the quartzite was removed in this way. Farmers 
sometimes remove bowlders by this process. 

Frost {Freezing and Thawing). — Water is usually found in 
crevices and the minute spaces between the particles which com- 
pose the rock. When this water freezes it expands and breaks the 
rock just as water freezing in water pipes breaks the pipes. The 




Fig. 95. — Oval Concretions 
Exposed by weathering of the weaker sandstone surrounding them. Near New Castle, 

Wyoming. 

effect upon the rock is the same as would be produced by driving 
minute wedges into each space containing water. This action is 
sometimes called the "wedge work" of ice. 

The process of freezing and thawing is more effective in weather- 
ing porous rocks, particularly those composed of large crystals, 
than compact rocks. In a dry though cold climate this action is 
of much less importance than in a moist, cold climate. 

The obelisk now in Central Park, New York City, stood for 



228 



PHYSIOGRAPHY 




Fig. 96. 



-Ripple Marks, Formerly Wkstd (Volts Trading Post, New Mexico) 
By permission of Oliver Lippincott. 



3,000 years near the mouth of the Nile in Egypt, yet when it ar- 
rived in the city the inscriptions on it were finely preserved. In 
a short time freezing and thawing had weathered it to such an 
extent that it became necessary to treat the surface of the obelisk 
with paraffine to fill the pores and keep the water out. Fig. 93 
shows the extent to which the rock forming the top of Pike's Peak 
has been broken by this action. 

Wind. — The sand blast, a device which blows a stream of sand 
against objects, is widely used as a means of cleaning the outside of 
stone buildings, removing rust from metals, etching glass, and similar 
processes. Wind-blown sand is a natural sand blast; it loosens par- 
ticles from exposed surfaces of rock, and adds them to the mantle rock. 

Window panes in houses, in certain localities on Cape Cod, are 
abraded by wind-blown sand and their transparency destroyed; and 
in regions of strong winds pebbles are worn into triangular shapes and 
even perforated. 

Plants and Animals. — The roots of plants find their way into cracks 
in rocks and as they grow larger exert great pressure on the rock, often 



THE MANTLE ROCK 



229 



breaking off large pieces. Roots of trees growing near a city sidewalk 
frequently illustrate this action by raising or breaking the walk. The 
decay of vegetable matter supplies acids which act vigorously on cer- 
tain minerals. 

Earth worms, moles, ants, and other animals living in the ground 
bring much soil to the surface, exposing it to the air, and thus play an 
important part in changing insoluble minerals into the soluble form suit- 
able for plant food. They also aid in the distribution of air and ground 
water through the tunnels and holes which they make. 

Gravity assists in weathering rock by removing loosened fragments 
from steep rock walls, thus exposing fresh surfaces to the air. 

Weathering Below the Surface. — Certain kinds of weathering 
take place below the surface, but it is in general much less rapid 
than on the surface; indeed, one foot of impervious soil has fre- 
quently been found to have quite perfectly preserved the polish 
and the scratches given the bed rock by continental glaciers. In 
porous mantle rock weathering certainly takes place at consider- 
able depths. This is proved by the thick deposit of residual man- 
tle rock which overlies some deposits of granite and other durable 
rocks. 




Fig. q7. — Sandstone Undercut by Wind-blown Sand (Banner County, Nebraska) 



230 



PHYSIOGRAPHY 



Residual Mantle Rock. — Some portions of the mantle rock re- 
main in the position in which they were formed and such deposits 
are called residual mantle rock. All residual mantle rock is a 
product of the weathering of the bed rock below it, and consists 
only of such materials as can be formed from the bed rock by 
the processes of weathering. The gravel and stones scattered 
through the deposit are all like the bed rock except as they show 
various stages of decomposition. The upper layers of residual 




Fig. q8. — Diagram of Residual Mantle Rock 



mantle rock consist of smaller and more perfectly decomposed 
particles than the layers below them, because these upper layers 
protect to some extent those below them. There is usually a 
gradual increase in size and angularity of the fragments as we de- 
scend, as indicated in Fig. 98. 

A change in the character of the bed rock is at once indicated 
by a change in the nature of the mantle rock, and it is not usual 
to find large areas having the same kind of residual mantle rock. 

Deposits of Vegetable Matter. — During the last stages of the 
destruction of a pond or a lake vegetable matter accumulates 
more rapidly than the other materials which fill them, and the 
swamp thus formed is often a bed of plant fibre that is quite free 
from earthy matter and that burns well when dried. Peat is 
formed in this way. It consists chiefly of the remains of mosses 



THE MANTLE ROCK 



231 



and marsh grasses which are but slightly decomposed. Many 
ponds and marshes illustrate a stage in the formation of peat, 
and many peat bogs are found in the New England States, in 
New York, and in many other parts of the United States. The 
peat bogs of Ireland are well known and very extensive, one of 
them having an area of more than 600 square miles. 




Fig. 99.— Transported Mantle Rock 
Bluff Point, N. Y. 

The Dismal Swamp of Virginia, and the million acre swamp of 
the Kissimmee Valley of Florida, are examples of large deposits 
of a similar nature in the United States. These deposits of partly 
decomposed vegetable matter are a part of the mantle rock and 
are like the residual mantle rock in that they have not been re- 
moved from the locality where they were formed. 

When exposed to the air vegetable matter decays and its con- 
stituents pass into the air, but when under water it loses its volatile 
constituents and gradually approaches more and more nearly a 
pure form of carbon. 

The mosses that form these deposits grow on the surface and die 



232 PHYSIOGRAPHY 

beneath, thus raising the surface so that it sometimes rises above 
the surrounding land or even climbs an adjoining hillside as in 
the "climbing peat bogs." 

Transported Mantle Rock is that which has been carried to the 
location where it is found by some natural agency. Its composi- 
tion, as a rule, bears no relation to that of the underlying bed rock, 
and is a mixture of fragments of many kinds of bed rock. Since 
certain agents which transport rock-waste act over large areas, we 
sometimes find deposits of transported mantle rock of quite uni- 
form composition and structure extending over thousands of square 
miles. The deposits of all large rivers illustrate this fact. 

Transportation of Mantle Rock. — Five agents are chiefly respon- 
sible for the transported mantle rock. 

1. Rivers. — Every muddy stream is actively engaged in the work 
of transporting mantle rock, and each stream has a burden in prog- 
ress toward its mouth that is measured by the extent of the bottom 
lands along its valley and the depth of the transported mantle rock 
that forms the bottom lands. Mantle rock that has been trans- 
ported by streams is called alluvial mantle rock. 

2. Glaciers. — The glaciers carry mantle rock slowly, but the size 
of the particle carried is not limited by the velocity, as it is in the 
case of rivers, and the total load that a glacier can carry is limited 
only by the amount that it can get. The greater part of the trans- 
ported mantle rock in the northern United States and in north- 
ern Europe is glacial mantle rock. 

3. Wind. — The presence of dust in the air is a familiar fact in 
every household; it settles on everything that air reaches. No 
building is so tall that the upper story rooms never need dusting, 
and no mountain is so high that its snows are free from dust. 

In the Sahara a sand storm sometimes overwhelms caravans, and 
even when not fatal involves them in great confusion and danger. 

In the Missouri Valley during low water great clouds of sand 
and dust are picked up by the winds and carried many miles. 
In the arid regions of the Southwest it is claimed that the dust 
storms are as dangerous as the blizzards of the Northwest. 



THE MANTLE ROCK 233 

Volcanic eruptions sometimes project great quantities of ash or 
volcanic dust (finely divided lava) into the air. The finer par- 
ticles of this dust are carried great distances; indeed, it is be- 
lieved that the dust projected into the air during the great erup- 
tion of Krakatoa in 1883 was carried several times around the 
earth and that some of it remained in the air for three years. This 
is probably the only way in which material from the land adds to 
the deposits forming in mid-ocean. 

4. Gravity. — Avalanches and landslides are well-known illustrations 
of transportation through the action of gravity which occasionally moves 
great masses of mantle rock. A recent landslide in British Columbia 
removed a large section of a mountain, buried a town located in an ad- 
joining valley, and portions of the mountain were carried some distance 
up the opposite side of the valley. 

A disastrous avalanche occurred February 27th, 1910, in northern 
Idaho. It buried the mining towns of Mace and Burke, with great loss 
of life and destruction of property. On March 1st, 1910, a train on the 
Great Northern Railroad was swept from the tracks by an avalanche 
which buried the track beneath a mixture of snow and earth. The ac- 
cident occurred at Wellington, Wash., near the summit of the Cascade 
Mountains. 

In connection with the ground water gravity moves the mantle rock 
slowly down slopes, sometimes breaking off pieces of inclined strata 
over which it passes, and opening the layers so that air and water may 
circulate more freely. This action is known as creep. 

Mantle rock that has been transported by gravity is called colluvial 
mantle rock. 

5. Waves. — Between the breakers and the shore line water 
dashes up the beach from every incoming wave and carries so much 
of the beach sand with it that the water usually looks muddy. If 
the sand is white and free from clay, the water becomes clear at 
the instant that the shoreward motion ceases, to become muddy 
again as it gains velocity during its return. This latter motion 
follows the laws which govern motion down an inclined plane; it 
moves in the direction of the slope of the plane and increases .its 
velocity at a rate which depends upon the slope of the beach. 
This return motion, the undertow, carries the finer particles of the 
beach deposit with it. 



234 PHYSIOGRAPHY 

When the wave is oblique to the shore line the to and fro motion 
of the water between the breakers and the shore is not along the 
same line as it is when the waves are parallel to the shore. This 
backward and forward motion transports the beach materials 
slowly along the shore. The amount of material transported in 
this way increases as the waves become more oblique and reaches 
a maximum when the wind is parallel to the shore. 

Deposition. — The agents that transport mantle rock deposit 
it as they lose carrying power and form physical features that differ 
so widely in shape and structure that, in most cases, the agent that 
transported a given deposit may be readily determined. 

i. Alluvial Deposits. — The sediments carried by streams are 
quite perfectly assorted, giving us layers of mud, silt, sand or gravel 
in the various deposits, but the stratification is very irregular. A 
layer of clay may be found a given distance below the surface at 
one point, and ioo feet away gravel may take its place. Such 
changes are due to the fluctuations in volume which vary the trans- 
porting power of the stream and which often wear away portions 
of a deposit, afterward filling the depression thus formed with 
material that differs from that removed, and breaking the continu- 
ity of the layers. The gravel of the deposits consists largely of 
rounded pebbles of the more durable rocks. The physical features 
thus formed have a nearly level surface. 

Flood plains, as the valley flats which border many streams are 
called, have the characteristic irregular stratification mentioned. 
They are usually somewhat higher along the margin of the stream 
than farther away and often slope gently down stream, following 
the river profile. 

Deltas. — The upper and lower beds of a delta consist of nearly 
horizontal layers of fine material and the middle portion of diagonal 
layers of coarser particles. The middle layers are formed by the 
material rolled along the bottom of the stream. 

Fans and Cones. — These are ordinarily semi-circular deposits 
with very imperfect stratification. They occur where a stream 
leaves a gorge or ravine having a steep slope and flows over a low- 
land of more gentle slope. The coarsest material is found where 



THE MANTLE ROCK 235 

the most abrupt change in slope occurs, that is, at the mouth of 
the gorge. 

2. Glacial Deposits. — The deposits formed by a glacier are always 
unassorted and unstratified, and they consist of many kinds of 
rock. Fragments of weak rocks, like shale, are found in them. 
The pebbles are angular instead of rounded and their surfaces are 
rough like freshly broken stone, except where one has been smoothed 
and flattened by contact with the rock over which the glacier 
passed. Unlike the river sediments, glacial deposits contain little 
decomposed rock; even the smallest particles are ground rock 
rather than decomposed rock. 

Among the more important features formed by glaciers is the 
terminal moraine described on page 337. Its surface is irregular, 
with mounds and hummocks associated with irregular depressions. 
Level sky lines are conspicuous by their absence. 

The drumlin described on page 344 is an oval hill of bowlder 
clay and was deposited under the ice. Deposits formed under the 
ice are sometimes composed of rock fragments and bowlders im- 
bedded in a tough clay. This is called bowlder clay. 

3. Aeolian Deposits. — Inasmuch as the wind holds the fine 
particles in suspension longer than the coarse, moving air deposits 
the coarse and fine particles in different places. This results in 
layers made up of particles which within certain limits are uniform 
in size and weight. The assorting is much less perfect than that 
of water, and the conditions causing deposition of a stratum of a 
given character are generally less permanent. The velocity 
of the wind is proverbially inconstant and every change alters the 
size of particle deposited; but deposits formed by wind show dis- 
tinct and characteristic stratification. 

Obstructions are effective in determining the location of the 
coarser particles to a somewhat greater extent even than they are 
in determining the location of snowdrifts, because the greater part 
of the sand is carried in the lower layers of the air. A rather larger 
proportion of such deposits, therefore, will be found about ob- 
structions. The deposit itself becomes an obstruction of increasing 
importance. Such hills of wind-deposited sand are called sand dunes. 



236 



PHYSIOGRAPHY 




Fig. 100. — Sand Dune Advancing Over Trees (Dune Pare, Indiana) 
Note the steep slope of the lee side. 



Sand Dunes. — The typical sand dune has a much more gentle 
slope on the windward side than on the leeward. This is true of 
even the smallest deposit, such as that formed about a chip; the 
sand grains carried by the air strike the chip, lose velocity and 
drop or bound back, piling up on the windward side until the pile 
forms an inclined plane up which the wind can roll grains of sand. 

Standing beside a small dune when a strong wind is blowing one 
sees the sand moving up the windward slope, streaming over the 
crest, and falling upon the leeward slope, which from time to time 
adjusts itself to the proper angle by miniature landslides formed 
where it has become too steep. The angle at which such a slope 
will come to rest is called the " angle of repose," and varies with 

the size and shape of the par- 
ticles. 

FiG.tox.-mAGRAMorSANDDu^T Dunes are numerous along 

Arrow shows wind direction. Coasts, because sand is Com- 



THE MANTLE ROCK 237 

monly found there. They are more likely to be formed by on-shore 
than by off-shore winds (why?); and they are more common on the 
east side of bodies of water in the prevailing westerlies and on the 
west side of similar bodies in the trade wind belt, than on the oppo- 
site sides. For example, dunes of great height occur on the east 







1 




, a * [/ 


.4 f - 


... jr s 




. 




1 


■ JBfvfl^^^ 








'; J ^B 


im^K 









Fig. 102. — Tree Stumps Uncovered as a Sand Dune Migrated (Dune Park, Ind.) 
Note the gentle slope of the windward side. 

side of Lake Michigan, as at Grand Haven, Mich., and very 
few are found on the west side of the lake. 

Dunes also abound in deserts and in the semi-arid regions of the 
United States, sometimes reaching the height of several hundred 
feet. In regions subject to nearly constant winds the removal 
of sand from the windward side and its deposition on the leeward 
side causes the dunes to migrate slowly in the direction of the 
prevailing wind, sometimes burying buildings and forests. 

Dust and sand grains are supported in the air by irregular ascending 
currents, both convectional and forced. In the absence of such support- 



2 3 8 



PHYSIOGRAPHY 




Fig. 103. — Soil above Mantle Rock (Portland, Oregon) 
The mantle rock consists of sand above and gravel below. 



ing currents the larger particles settle quickly, but the weight of the 
smallest particles is so slight that it is nearly balanced by the resistance 
of the air to motion and these particles settle very slowly. 

The Loess. — In Kansas and other western States, in Europe, and 
notably in China, there are deposits called loess, consisting of particles 
larger than those of clay but smaller than those of sand. Their origin 
is in dispute, but there seems to be good evidence that a part of it, at 
least, is a wind deposit. It is without the distinct horizontal stratifica- 



THE MANTLE ROCK 



239 



tion of aqueous deposits and approaches consolidated rock in its ability 
to stand with a nearly vertical face. Some deposits of loess are 1,000 
feet in thickness. 

Volcanic Dust. — In Kansas and Nebraska there are beds of vol- 
canic dust three feet thick which cover large areas and which are 
hundreds of miles from either active or extinct volcanoes. Pompeii 
was buried to a depth of about 20 feet by such a deposit. 




Fig. 104. — Stratified Clay (Haverstraw, N. Y.) 
Used chiefly for bricks. 

4. Colluvial Deposits.— The most numerous of these deposits is 
the talus slope that forms at the foot of ledges of bare rock and 
that eventually covers the ledge with mantle rock. 

5. Shore Deposits. — The assorting action of the waves deposits 
layers of clay composed of particles of remarkable uniformity in 
size. Only the harder and more durable minerals remain on the 
beach, and as these grow smaller they are carried out to deeper 
water. This is why beach sand is chiefly quartz fragments. Quite 
sizable pebbles may be mixed with the sand grains, but vigorous 
wave action completely removes the fine particles and often leaves 
the sand white. Beach pebbles are generally quartz pebbles and 
are smoothed and rounded. 



24° 



PHYSIOGRAPHY 



Useful Materials from the Mantle Rock. — In addition to the 
economic importance of the mantle rock as a whole, it is of much 
importance as a source of supply of clay, sand, gravel, marl, peat, 
and many materials used in the arts. 

Clay occurs in very large quantities, is widely distributed, is of 
various degrees of purity, and is suitable for many uses. The 
purest clay, kaolin, is used in manufacturing the better class of 




Fig. 105. — Clay Pit (Near Vancouver, Wash.) 
B, gray brick loam. P, blue clay used for terra cotta. 

porcelain. The less pure varieties are used in making chinaware, 
pottery, terra cotta, tiles, drain tiles, and bricks. Fire clay, used 
in the manufacture of furnace and stove linings, owes its ability 
to withstand high temperatures to the absence of lime and such 
alkaline substances as act as a flux. 

The clay products manufactured in the United States are valued 
at about $160,000,000 a year. 

Sand is used in making glass, mortar, and cement. It is also 
used in molding metals and as an abrasive. The sand used for 
these purposes yearly is valued at about $15,000,000. 



THE MANTLE ROCK 241 

Gravel is used in roofing, in concrete, and in road building. 

Marl is used as a fertilizer, in making certain kinds of bricks, 
and in making Portland cement. 

We obtain from the mantle rock of the United States more than 
half a million dollars worth of these necessary materials every 
working day of the year. 

THE SOIL 

Economic Importance. — The upper and fertile portion of the 
mantle rock is called soil. It differs from that below it, which is 
called sub-soil, chiefly in the greater quantity of decaying animal 
and vegetable matter called humus and in the large number of 
bacteria which it contains. 

Agriculture has been the most important means of support from 
the earliest times, and the progress of the early nations depended 
in a more marked degree even than that of modern nations upon 
the fertility of their soil and their skill in cultivating it. In the 
United States the yearly value of the direct and indirect products 
of the soil exceeds $7,000,000,000, or more than three times the 
total value of all the mineral products. 

Fertility.— Soils differ greatly in fertility from place to place, 
because of unlike composition and unlike texture. 

Composition. — All plants require nitrogen, potash, and phos- 
phorus, and these elements of plant food must be natural constit- 
uents of the soil or must be supplied artificially to make the soil 
fertile. The soils of residual mantle rock contain only such of these 
elements as were in the rock from which they were formed. Granite 
and kindred rocks are usually rich in potash and deficient in phos- 
phorus, though some of them contain the latter. A pure lime- 
stone usually contains an abundance of phosphorus, derived from 
shells, but is deficient in potash, and soil formed by its decomposi- 
tion would be similarly deficient. A shaly limestone, like that at 
Trenton, N. Y., contains both phosphorus and potash. The fa- 
mous " Blue Grass Region " of Kentucky has a soil formed by the 
decay of such a limestone. A pure sandstone contains neither 
phosphorus nor potash, and would form an unproductive soil; but 



242 PHYSIOGRAPHY 

sandstones containing many fossils produce a soil containing phos- 
phorus. The unproductiveness of the sandstone soils in Kentucky- 
is in marked contrast with the fertility of the " Blue Grass Region." 
Transported soils are likely to be more fertile than residual soils 
because the processes of transportation tend to grind them finer, 
to mix the soils of different localities, and to increase the amount 
of organic matter in them. Such soils necessarily differ among 
themselves as the agents by which they were transported and 
deposited differ. 

Texture. — The physical condition of the soil is fully as impor- 
tant to its fertility as is the chemical composition. If the particles 
composing it are very small the amount of water retained in the 
fine capillary passages between them will be large, and because 
of the high specific heat of water the soil will warm slowly. Such 
soils are "cold" and "late." 

Fine grained soils do not absorb so much of the rainfall as coarse 
grained soils, and the run-off on the former is greater in propor- 
tion than on the latter type. The size of the particles composing 
the soil also determines the nature of the plant's water supply, and 
hence the ability of the crop to withstand drought. If they are 
too large, water is not lifted a great distance by capillary action, 
and plants die when the water table is too far below the surface. 
If they are too small water rises too slowly, with the same 
result. 

The situation of soil controls the accumulation or loss of humus 
and the finer particles of the soil, as well as the available plant 
food. On steep slopes the swift flow of the run-off above the sur- 
face and of the ground water below causes them to wash away the 
smaller and more perfectly decomposed particles, and to dissolve 
and remove the soluble parts of the soil from which plants derive 
their food. Soils on such slopes are always less fertile than soils 
of the same origin on more gentle grades. 

Fertility of soil requires something more than plant food. It 
requires water in the right amount and at the right time; it requires 
heat; it requires air, which must be distributed through the soil 
and must be renewed as it is exhausted; and finally, it requires 



THE MANTLE ROCK 243 

tillage, which contributes mellowness, facilitates the renewal of the 
air supply, and conserves the supply of moisture. 

Origin. — The flood plain of the Mississippi and the valley of the 
Sacramento in California have alluvial soils of great fertility. 
The valley of the Red River of the North in North Dakota has 
lacustrine soil, deposited on the bottom of a former lake, and is 
one of our great wheat growing regions. The region covered by 
the continental glacier has a glacial soil. It is less uniform in its 
character than either alluvial or lacustrine soils. On Long Island 
and Cape Cod it is sandy, whereas the New England States 
have many clay soils which are parts of the ground moraine. The 
large deposit of //// in northwestern Ohio provides a soil that 
is more fertile than the residual soil of the southeastern part of 
the State, but is less fertile than the alluvial soils bordering the 
Ohio River. 

Types of Soil. — The common classification of soils as sands, 
loams, and clays is based upon the physical structure or texture 
of the soil rather than upon its chemical composition. It is true 
that coarse sands are usually composed chiefly of quartz grains, 
and that clays contain a larger percentage of kaolin than either 
sand or loam; but the distinguishing characteristics such as plas- 
ticity and ability to hold moisture depend chiefly upon the size of 
the particles composing the soil. 

Sands are composed of particles between 1 mm. and .05 mm. in 
size. Their distinguishing characteristic is their want of coherence 
when dry, and this characteristic is possessed equally by the sand 
composed of quartz fragments, with which we are all familiar, and 
by the sand found about coral islands, which is made up of frag- 
ments of coral and shells. 

Sandy soils are porous and well drained; they permit free circu- 
lation of the air, but are likely to suffer from drought. They are 
classed as " early " and " warm " soils, and if they are not too 
coarse yield excellent crops of garden truck and potatoes. 

Clay is composed of particles less than .005 mm. in size. It is 
plastic when wet, shrinks on drying, but retains the form given it 



244 



PHYSIOGRAPHY 



when plastic. It becomes impervious to water when puddled 
(worked with water to a thick paste). 

Clay soils permit very little circulation of the air. They are 
usually poorly drained and are therefore likely to be " drowned " 
in a wet season. They are less likely to suffer from drought than 
gravel or sand, but do not stand dry weather as well as loam. 
They are "cold" and "late" soils, but make good meadows. 

Loam. — This is a mixture of sand and clay containing enough 
coarse particles to make the soil mellow and to permit free circu- 
lation of air. It also contains enough fine particles to facilitate cap- 
illary circulation, but not enough to make the soil sticky in wet 
weather. Loams are well drained and therefore stand wet weather 
well. They also stand dry weather well. 

Silt is the term applied to deposits of river-borne sediments com- 
posed of particles between .05 and .005 mm. in size. 

Muck is a black soil formed in swamps and contains a large 
quantity of humus; hence it is rich in nitrogen. 

The following table shows the percentage of particles of various 
sizes to be found in some of the types of soil : 



Size Particles 


Barren Sand 


Coarse Sandy 
Loam 


Clay Loam 


Clay 


Clay, .005 mm 


83.6 
5-4 
1.8 


75.6 

7.2 

11. 7 


48.1 
24-3 
18.5 


7.6 
32.2 
42.2 





The sandy loam described in the table is an early and warm soil 
that is well drained and stands drought well. The clay contains 
so large a percentage of the finest particles that it is very wet 
during a rainy season, and supplies water to plants so slowly that 
they would be parched during a drought. 

The Department of Agriculture at Washington publishes the 
following table of the percentage of each size of particles in typical 
soils for certain crops: 



THE MANTLE ROCK 



245 





Truck 


Corn 


Wheat 


Grass 


Bright 
Tobacco 


Heavy 
Tobacco 


Barren 
Clay 


Gravel, 2-1 mm. .. 










3-°9 


1 . 12 




Sand, 1-.25 mm.. . 


6-34 


2.80 


i-95 


. 21 


28.90 


3-19 


.29 


Fine Sand, 
















.25-05 mm 


81 .92 


43.06 


42.90 


11.47 


49.68 


5- 73 


10. 20 


Silt, .05-.005 mm. . 


8.17 


40.90 


32-13 


23.69 


21 .41 


44.98 


36.98 


Clay, 
















.005-. 0001 mm.. 


2.80 


10. 10 


23.78 


51-75 


4.80 


35-24 


50.02 



The typical soil for vegetables or garden truck seems to be warm, 
sandy, and well drained; that for corn a sandy loam, and that for 
wheat a clay loam. 

QUESTIONS 

1. Is all rock properly called stone? Why? 

2. What two forces distribute water through the mantle rock? 

3. Show that the mantle rock tends to keep the flow of streams uni- 
form. 

4. Why is spring water better for drinking purposes than surface 
water? 

5. How does the action of the chemical agents of weathering differ 
from that of the mechanical agents? 

6. Under what climatic conditions is the action of freezing and thaw- 
ing most effective in disintegrating rock? 

7. What kind of soil retards weathering below the surface? 

8. Dust is always present in the air, yet it is always settling. How is 
it supported in the air? 

9. Compare residual soils formed from granite with those formed from 
a pure limestone. 

10. What should result when rain falls on heated rock? In what 
two ways may igneous rocks become surface rocks? 



CHAPTER XVIII 
THE BED ROCK 

Rock-making Minerals. — The consolidated rock of the litho- 
sphere was formed in various ways and is of many kinds, but with 
the exception of coal and a few similar deposits of animal or of 
vegetable origin it is all composed of mineral matter. Much of it 
consists of minerals in crystalline form, and the rest, with the ex- 
ception of coal, consists either of fragments or decomposition-prod- 
ucts of minerals, or is a fused mass of mineral matter. 

The term mineral was originally used to designate a substance 
found in a mine, hence something found in the rocks as distin- 
guished from animal and vegetable products. 

A mineral is a natural substance not of obvious organic origin and 
having definite chemical and physical properties. 

The durability and economic value of building stones depends 
to a large extent upon the physical properties of the minerals 
from which they were formed. The following are the rock-making 
minerals of most frequent occurrence: 

Quartz. — This is the hardest of the common minerals. It is 
harder than glass, is almost infusible, and is not affected by com- 
mon acids. It is quite brittle and the broken surface is curved 
like the surface of a shell. It has no cleavage, is of glassy luster, 
and occurs in many colors. When in crystals it forms six-sided 
prisms, terminated at one or both ends by six-sided pyramids. 

The gems, amethyst, carnelian, opal, onyx, and bloodstone, be- 
long to the group of minerals of which quartz is the type and have 
almost identical properties. Flint, another similar mineral, was 
of great importance to prehistoric man because of the sharp cutting 
edge of broken pieces. From this substance he fashioned his cut- 
ting implements such as knives, awls, spearheads, and arrow points. 



THE BED ROCK 247 

Later the flint-lock musket was used in the American Revolution, 
and it is reported that many of the guns used during the Civil 
War were altered to "flint locks," and sold to the savage tribes in 
Africa. 

Feldspar is first in importance as a rock-making mineral. It 
occurs in a variety of colors, commonly pale pink, yellow, or white, 
but sometimes gray, blue or iridescent. It is nearly as hard as 
quartz but cleaves easily in two directions, giving flat reflecting 
surfaces. When exposed to moist air containing carbon dioxide, 
or to infiltrating water containing carbon dioxide or other acids, its 
luster is quickly lost and it soon crumbles into a soft clay, called 
"kaolin." Because of its ready cleavage and its lack of permanence 
under natural conditions, feldspar is not a durable mineral, and 
most of the clay and the mud rocks of the earth are chiefly products 
of its decomposition. Feldspar and kaolin are used in making 
porcelain and china, and feldspar is valuable as a fertilizer. 

Mica is familiar to everyone in the misnamed isinglass used 
in stoves. Its most important properties are its perfect cleavage 
into very thin, elastic leaves which have a pearly luster, its ability 
to withstand high temperature, and the resistance it offers to the 
passage of currents of electricity. It usually occurs in rocks in 
rather small sheets or scales, but sometimes large masses are found 
which furnish large sheets. White and black are the common 
colors. Mica is very soft, and rocks containing an excess of it 
are easily broken. It is used as an insulator in electrical appara- 
tus, also in stove doors, lamp chimneys, and wall papers. 

Calcite. — When pure and crystallized, calcite is a transparent, 
colorless crystal which cleaves in three directions, making oblique 
angles with each other. It is much softer than quartz ai.d is easily 
scratched with a knife. Its effervescence with dilute acid and its 
double refraction distinguish it from other common minerals. 

Calcite is one of the most abundant minerals, ior it forms the 
basis of limestone, one of the commonest rocks. It is dissolved 
by water containing carbon dioxide in solution; therefore, lime- 
stone and other rocks containing much calcite are worn away by 
rain water. 



248 



PHYSIOGRAPHY 



Structure of the Bed Rock. — When one visits a stone quarry or 
a rocky ledge he often finds that the rocks are arranged in parallel 
layers like those shown in Fig. 106. The layers may differ in 
color and in kind of rock, or there may be many layers of the same 
kind; some of the layers may be less than an eighth of an inch in 




Fig. 106.— Stratified Rock 
Near Engineer Mountain, Cal. Beds of hard sandstone or limestone alternate with shale. 
The pass of Coal-Bank Hill is shown on the right. 



thickness and others many feet thick. The layers are commonly 
horizontal, though sometimes upturned like those shown in Fig. 
114. These are the " bedded " or stratified rocks. They are chiefly 
sandstones, limestones, and shales, but sometimes layers of coal, 
conglomerate, or iron ore are found. 

In exceptional localities, particularly in mountainous regions, 
we sometimes find massive rocks, as shown in Fig. 108. These are 
the "crystalline" or unstratified rocks. They are more apt to ap- 
pear on the surface in mountains, but are found everywhere below 
the stratified rocks when we dig deep enough. The bed rock con- 
sists, we must conclude, of a great mass of unstratified rock which 



THE BED ROCK 



249 




is covered in most places by the beds of stratified rock. As a rule, 
the unstratified rock is reached by borings less than a mile deep, 
but in some places the stratified rocks are much thicker than that. 
In the Colorado Canon, more than 8,000 feet of consecutive 
stratified rocks are exposed at one point, but at the bottom of the 
canon unstratified rock is found. 



250 



PHYSIOGRAPHY 




Fig. 108. — Unstratipied Rock. Yosemite Valley 
Copyright by Underwood and Underwood. 



THE BED ROCK 251 

Origin of the Bed Rock. — Portions of the bed rock show that 
they assumed their present form on cooling from a molten state and 
are therefore called igneous rocks. Modern lavas belong to this 
class, but form a very small part of it. Much of the unstratified 
rock which underlies the stratified rock is of igneous origin. 

Other portions of the bed rock accumulated as sediments in some 
body of water. These are called sedimentary rocks. They are always 




Fig. 109. — A Lava Flow With Unbroken Surfaces (Hawaii) 

stratified, and so large a portion of all the stratified rock was formed 
in this way that it is customary to classify those that accumulated 
on land with the sedimentary rocks. Sedimentary rocks are made 
up of products of the disintegration and decay of former rocks ; in 
other words, they consist of mantle rock which has been assorted, 
accumulated in layers and consolidated. 

A third portion of the bed rock has been so changed through the 
action of natural agencies as to give the rocks new properties. 
These are the metamorphic rocks. They are usually composed 
wholly or partly of crystals. 

The Igneous Rocks. — Every volcanic eruption contributes to 
the igneous rock of the surface. Some lavas come to the surface 



252 



PHYSIOGRAPHY 




in a very fluid state, and cooling quickly /form a volcanic glass 
called obsidian. Some lavas are mixed' with steam so thor- 
oughly as to form a very porous glassy rock called pumice. In 
some cases the explosion of steam and other gases separates the 

'Areas in which the top of the bed rock is metamorphic are shaded; those in which it is 
igneous are marked vv. The white area includes certain minor areas in which the origin of 
the bed rock is unknown, but with these exceptions it is sedimentary. 



THE BED ROCK 



253 



lava into fine particles, which fall as volcanic ash or dust. These 
different forms are due to the conditions of the eruption rather 
than to differences in composition of the lava. 

Classes. — Certain lavas, particularly at the bottom of a thick lava 
flow, show the beginnings of a crystalline structure. Many extinct 
volcanoes have been so eroded that lava, which was covered by thick 
beds of rock while it cooled, is now exposed, and these lavas are found 




Fig. hi. — A Granite Quarry 

to be perfectly crystallized. They differ from the lavas which cooled 
at the surface to such an extent that igneous rocks are divided into two 
classes: the eruptive class, or those which cooled rapidly at the surface, and 
the plutonic, or those which cooled slowly beneath a thick rock blanket. 

Granite is chiefly composed of quartz and feldspar, though mica 
is usually present. The minerals are often in such coarse crystals 
that they may be readily recognized without the aid of a lens, and 
they are irregularly distributed through the mass. Granite cooled 
very slowly and crystallized beneath a thick blanket of rock, which 
in many cases has been worn away. Granite is extensively used 



254 PHYSIOGRAPHY 

in buildings, in monuments, and in pavements. It is one of the 
more durable rocks. 

Sedimentary Rocks. — The beds of assorted mantle rock such 
as clay, sand, and gravel, when consolidated, form sedimentary 
rocks known as shale, sandstone, and conglomerate respectively. 
They were deposited in horizontal or nearly horizontal layers in 
some body of water, generally the ocean. Other sedimentary 
rocks were formed from material dissolved from the mantle or 
bed rock, carried to the ocean in solution, and recovered from the 
solution by plants and animals which absorbed the material from 
the sea water to form shells and skeletons. The most important 
rock formed in this way is limestone. 

Sandstone. — Quartz is the most durable of the common rock- 
making minerals, and fragments of quartz should therefore pre- 
ponderate in the coarser deposits of rock-waste formed in the sea. 
The sands of most shores are chiefly quartz fragments. Sandstone 
is a sand bed held together by some natural cement and may be 
recognized by its hardness, its rough feel, and the fact that it is 
composed of quartz grains. It is usually quite porous.* 

Conglomerate is a consolidated gravel bed composed of rounded 
pebbles, usually embedded in finer material. The mass is bound 
together by some mineral substance which forms the cement. 

Shale. — Decomposed fragments of feldspar and other minerals 
less durable than quartz reach the ocean in very small particles 
and settle to the bottom in quiet water, farther from the shore than 
the sand deposits. When consolidated, the beds thus formed are 
called mud stones or shales. 

Shale is so soft that it may be scratched with the finger nail. 
It splits readily into thin layers parallel to the planes of stratifi- 
cation, and it weathers quickly. It is not affected by acid, and when 
moistened has an odor of wet clay. Shale is useless for building 
purposes, but is used in manufacturing cement, terra cotta, and 

* Sandstone has a variety of colors. Perhaps the various shades of red and yellow are 
most frequently seen, but some specimens are nearly white, and impure sandstones often 
are blue or gray. It is used for grindstones, scythe stones, and in building. Shaly sandstones 
make excellent sidewalks. 



THE BED ROCK 



255 



bricks. Some of the black shales contain valuable oils like petro- 
leum which are recovered by distillation. 

Limestone. — The deposits of the calcareous parts of animals and 
plants which gather in the sea form limestone when consolidated. 
Some of these deposits are shell beds, like those that form 
coquina off the coast of Florida; others are beds of coral sand such 




Fig. 112. — A Limestone Quarry 



as form the beaches of coral islands ; and still others are made up of 
the harder parts of minute animals that inhabit the upper part of 
the sea even in mid-ocean. The latter deposits form chalk. 

Limestones formed near the continents are usually rendered 
impure by sand or mud brought by waves and shore currents. 
Pure limestone can only be formed in a region which does not 
receive such deposits. It may be formed in the deep sea, beyond 
the muds; or in shallow water, about coral islands; and in excep- 
tional localities about continents where sediment from the land is 
not being deposited. 



256 



PHYSIOGRAPHY 




Fig. 113. — Diagram Showing Relative Location of Sedi- 
mentary Deposits 



Some small deposits of limestone are formed on land by direct deposit 
from spring water which has lost its dissolved carbon dioxide, and can, 
therefore, no longer hold the limestone in solution. Calcareous tuff is 
thus formed. In some other cases limestone has been deposited in the 
beds of salt lakes through the evaporation of the concentrated sea water, 
but it is not believed that any of the important limestone deposits of 
the world have been formed in this way. 

Limestone effer- 
vesces when treated 
with a weak acid, 
and has about the 
same hardness as 
calcite. It is slowly 
dissolved by water 
containing carbon 
dioxide or acids de- 
rived from the decomposition of vegetable matter, and is there- 
fore one of the less durable rocks. 

About one quarter of the limestone quarried is used in making 
quick-lime and cement. The remainder is used in buildings and 
as a flux in smelting ores. 

Location. — Wave action assorts the sediments deposited in the 
sea and we find the coarsest material, the gravels which form 
conglomerates, nearest the shore; next beyond them come the 
sands which form sandstones; and next come the muds and clays 
which form shales. Beyond the muds are the calcareous deposits 
which form limestone. The diagram (Fig. 113) indicates the 
order in which these deposits would occur in a region where all 
of them were forming at the same time. 

When peat is buried beneath deposits which exclude the air it 
becomes more compact and gradually loses some of the constit- 
uents of woody fibre and approaches more and more nearly to a 
form of pure carbon. This is the way in which bituminous coal 
was formed, but the deposits from which it was formed contained 
the remains of ferns which grew to be large trees as well as palms 
and other forms of tropical plant life, in addition to the mosses 
and grasses which commonly form peat. 



THE BED ROCK 257 

Bituminous coal burns with much smoke and flame. This is 
due to the large amount of volatile matter which it contains and 
which makes it valuable in the manufacture of artificial gas. It 
has a dull black color and usually breaks along bedding planes 
parallel to the coal seam. 

Chemical Deposits: Rock Salt and Gypsum Beds. — When a 
salt lake dries up, or a body of water that has been isolated 
from the sea evaporates, all of the dissolved mineral matter is 
deposited, and an interesting assorting action of great importance 
to mankind accompanies the deposition. This assorting is due 
to the varying solubilities of minerals. In the case of sea 
water, gypsum, a very difficultly soluble mineral, begins to 
be deposited when 37% of the water is evaporated; common 
salt, a very soluble mineral, is not deposited until 93% is 
evaporated. Epsom salts is deposited after the common salt is 
removed, and after this certain other compounds are deposited. 
Some deposits of rock salt were thus formed; they are usually 
underlaid by beds of gypsum, and where the evaporation has been 
complete, as it was at Stassfurt in Saxony, there are overlying 
beds of magnesium and potassium compounds. Large chemical 
establishments exploit these beds, producing some very important 
fertilizers and many chemical products. 

Metamorphic Rocks. — Some of the metamorphic rocks are known 
to have been slowly formed from sedimentary rocks, others have 
been formed from igneous or other metamorphic rocks. The chief 
agents by which rocks are metamorphosed are heat, moisture, and 
pressure. 

Rocks have sometimes been metamorphosed by heat from a 
lava flow or a dike, and we may find layers of sedimentary rocks 
passing gradually into metamorphic rocks as we approach the 
lava, thus showing what kind of rock was metamorphosed and 
the cause of the change. Sandstones have been changed into 
quartzite under such conditions; shales and clays into slate and 
then into mica schist. Pure limestone has been changed into 
white marble, and shaly limestone into a marble containing such 
minerals as mica and garnet; and in several States along the 



258 PHYSIOGRAPHY 

Appalachian Mountains bituminous coal has been changed into 
a natural coke or into an anthracite. When large areas have 
been metamorphosed it is not always possible to determine the 
original form of the metamorphic rock. The changes produced by- 
pressure are of fully as much importance as are those due to heat. 
Pressure has caused the characteristic cleavage of slate, Fig. 114, 
and the foliated structure of gneisses and schists. 

Quartzite is a metamorphosed sandstone. The separate grains 
can easily be distinguished by the aid of a lens, but they are much 
more firmly cemented together than are those of sandstone, and 
the rock is less porous. The cement is silica, like the grains, and 
it has a shell-like fracture. Metamorphism seems to be destroy- 
ing its granular structure and restoring the properties of the 
quartz crystal. 

Marble is a metamorphosed limestone which in its typical form is 
quite perfectly crystallized. Pure limestone becomes white mar- 
ble. Colored marbles are due to impurities, such as iron. Marble 
is used for statuary, in the ornamentation of buildings, and was 
formerly much used for tombstones. 

Slate is metamorphosed shale. It is somewhat harder and more 
durable than shale, and it cleaves into thin layers having smoother 
surfaces than those of shale and quite independent of the original 
bedding of the mud deposit. Fig. 114 shows the cleavage lines of 
the slate across the bedding. The principal use of slate is for roof- 
ing. Small quantities are used for school slates and blackboards. 
It is also used in making imitation marble and as a support for 
electrical fixtures. 

Gneiss is composed of crystals of quartz, feldspar, and mica ar- 
ranged in parallel layers. Mica schist is composed chiefly of quartz 
and mica. Gneiss and mica schist may be of either igneous or sedi- 
mentary origin. They are widely distributed and of great extent 
in northern North America and in the central portion of mountain 
ranges. 

Anthracite Coal, seen in thin sections under the microscope, 
shows cellular structure, indicating that it is of vegetable origin; 
but it differs from bituminous coal in that it contains very little 



THE BED ROCK 



259 




Fig. 114. — A Slate Quarry 
Slatington, Pa. 

volatile matter and burns without the large amount of smoke and 
flame characteristic of bituminous coal. It is a hard, lustrous, 
dense substance which does not break along bedding planes as 
does soft coal, but has a shell-like fracture. 

Anthracite is found only in regions of disturbed and folded strata. 

Table. — The relation between the deposits of the mantle rock and the 
sedimentary and metamorphic rocks which they form when consolidated 
and metamorphosed is shown in the following table: 



MANTLE ROCK 


SEDIMENTARY ROCK 


METAMORPHIC ROCK 


Clay 

Sand 

Gravel 


Shale 

Sandstone 

Conglomerate 

Limestone 


Slate — schist 
Quart^ite 


Marl ) 

Shell Beds J 

Peat 


Marble 


Bituminous coal 


Anthracite coal 




(Graphite) 



260 PHYSIOGRAPHY 

ECONOMIC IMPORTANCE OF THE BED ROCK 

Metals. — Several metals occur in their pure state in the rocks 
and are called native metals. They are occasionally found in large 
masses but more frequently are scattered through the rock in small 
particles. Gold, silver, platinum, mercury, and copper are the 
principal native metals. 

Much of the metal of commerce is obtained from minerals in 
which the metal is combined with other elements. Such minerals 
are called ores if they yield a metal in "profitable quantities. The 
yearly output of our iron mines exceeds the total value of all the 
other metals together, and the value of our output of copper ex- 
ceeds that of gold and silver together. 

The principal ores of iron are compounds of iron and oxygen, 
from which the iron is obtained in a pure state by heating the ore 
with coke and limestone in a blast furnace. Most of the iron ore 
used in the United States comes from the Lake Superior region, 
but important mines are located in Alabama, New York, and 
Pennsylvania. 

The principal ores of zinc, lead, copper, and silver are compounds 
in which sulphur is combined with the given metal. Each of them 
must be treated by a more or less complicated chemical process 
to secure the pure metal. 

Coal. — About 300,000 square miles of land in the United States 
are underlaid with coalbeds. Not all of this is workable, because 
of its impurity or of the thinness of the layers, and the area at 
present producing coal is but a small fraction of the total. Very 
large areas in Alaska are also coal lands, but they are at present 
undeveloped. All varieties of coal are found in the United States, 
from the graphitic anthracite of Rhode Island, which burns with 
great difficulty, to the lignite of Texas, which retains much of its 
woody structure. 

The amount of coal mined in the United States last year ex- 
ceeded that of any other nation, reaching the total of more than 
500,000,000 tons. 

The map, Fig. 115, shows the regions in this country in which 



THE BED ROCK 



261 



coal is found. The coal is not all high grade, but recent progress 
in the construction of furnaces for low-grade coals and in the 
development of the " producer gas " process has made it possible 
to use low grade coals for heating purposes and also to produce a 
gas which is suitable for use in a gas engine and which works well 
under a Welsbach mantle. Now that we have learned to handle 




Fig. 115. — Coal Beds of the United States 

these coals, they have become immensely valuable. They usually 
have at least 60% of the fuel value of the high grade coals, and 
occur in regions where other grades of coal are expensive because 
of heavy freight charges. 

Petroleum and Natural Gas. — For the past fifty years we have 
been taking from the bed rock great quantities of petroleum and 
natural gas, which have accumulated during the ages. 

The value of these products secured during 1907 was about 
$173,000,000. 

In all of the oil fields old wells have ceased to produce or have 
diminished their output, showing that petroleum is an accumula- 
tion and that the supply is not renewed as fast as it is removed. 
We are rapidly using up the great store, and unless new fields are 



262 



PHYSIOGRAPHY 



n 




• = ■ 






'"■■'■ 



Fig. ii6. — A Group of On. Wells 



discovered men will have to learn before many generations to 
do without petroleum and natural gas. The most important oil 
fields are in western Pennsylvania and in Ohio. The Oklahoma 
field has recently come into prominence, and Kansas, Illinois, 
Texas, California, Colorado, and West Virginia each produces oil. 




Fig. 117. — Distribution of Petroleum and Natural Gas Fields in the United States 



THE BED ROCK 263 

Cement. — The manufacture of Portland cement has become an 
important industry in the United States. In 1885 the yearly out- 
put was valued at about $3,500,000; at the present time it exceeds 
$50,000,000. It is made by burning certain proportions of lime- 
stone or marl with clay or pulverized shale. A hard and durable 
artificial stone is made by adding water to a mixture of cement 
and sand. It is now being used extensively in sidewalks, buildings, 
and other structures. 

Stone. — Building stones are so widely distributed that almost 
every locality in the United States supplies its own demands. It 
is only when some unusual requirement, such as a particularly 
large-sized piece or a certain shade of color, is made that stone is 
shipped long distances. This condition is due to the willingness 
of most builders to use the stone at hand without regard to its 
durability, rather than to the universal distribution of good build- 
ing stone. 

Hundreds of " brown-stone " buildings of New York City show 
the lack of durability of this sandstone. Many of them have 
been treated with various solutions to fill the pores and protect 
the stone, but no satisfactory process of protecting poor stone has 
yet been devised. Probably the best known process is to saturate 
the exposed portion of the stone with a solution of water-glass, 
several applications being necessary. When this is dried a solu- 
tion of calcium chloride is applied. 

The following estimate of the " life" of different kinds of building stone 
in the climate of New York City is given in one of the volumes of the 
Tenth Census of the United States. By life of a stone is meant the 
period after which the decay becomes so offensive to the eye as to de- 
mand repair or removal: 

Life in years. 

Brownstone — coarse 5 to 15 

Brownstone — fine, compact 100 to 200 

Sandstone — best, silicious cement. . 100 to many tenturies 

Limestone — coarse 20 to 40 

Marble — coarse 40 

Marble — fine 50 to 200 

Granite 75 to 200 

Gneiss 50 to many centuries 



264 PHYSIOGRAPHY 

In value of stone quarried the largest yearly return is obtained from 
limestone. This is doubtless due to the many uses to which it is adapted. 

Mineral Fertilizers. — The most important mineral fertilizers 
found in this country are the rock phosphates of Tennessee, South 
Carolina, and Florida. These rocks owe their value chiefly to the 
great number of bones of mastodons and other animals of which 
they are largely composed. They are efficient fertilizers of lands 
needing phosphorus. Sodium nitrate (Chile saltpeter) occurs in 
beds several feet in thickness in the northern part of Chile, and 
is also found in Humboldt County, Nevada, and in California. 
It supplies about 15 per cent of its weight of nitrogen. One of 
the most valuable mineral fertilizers used to supply potash is salt- 
peter. Saltpeter is formed abundantly in certain soils in Spain, 
Egypt, and Persia, and is formed in considerable quantities in the 
soil of many caves in the Mississippi Valley. It yields about 45 
per cent of its weight of potash, and 13 or 14 per cent of nitrogen. 

Minerals. — Only a few of the many minerals found in the bed 
rock can be discussed here. For descriptions of others and for 
more complete accounts of those mentioned below, the student 
must be referred to works on Mineralogy and to such works as 
the " Mineral Resources of the United States," published annually 
by the United States Government. 

Graphite, commonly called " black lead," is pure carbon, being 
identical in composition with the diamond. Extensive graphite 
mines are located at Ticonderoga, N. Y., and in several Western 
States, but the quality of American graphite is not equal to that 
found on the Island of Ceylon and in Siberia. Graphite is a soft 
mineral with a metallic luster, and derives its name from its prop- 
erty of leaving a mark on substances. It is used in making lead- 
pencils, crucibles in which to fuse metals, stove polish, electric 
light carbons, and as a lubricant. 

Sulphur occurs uncombined about extinct as well as active vol- 
canoes. In the United States it is mined in Nevada, Utah, Cali- 
fornia, and Louisiana. Large quantities come from Vesuvius and 
other active volcanoes. It is extensively used in manufacturing 



THE BED ROCK 265 

gunpowder, fireworks, matches, and sulphuric acid. It is also 
burned to produce bleaching and disinfecting agents. 

Gypsum. — Like calcite and common salt, gypsum is one of the 
minerals brought down to the sea in solution, but it is not absorbed 
from the sea water by plants and animals, as calcite is. For this 
reason it is found chiefly in beds formed by the evaporation of salt 
water, and is usually associated with salt. Important mines are lo- 
cated in western New York, at Alabaster and Grand Rapids, Mich- 
igan, and in several of the Western States. It is not affected by 
acid, is softer than calcite, and has not the glassy luster of calcite. 
It is used as a fertilizer and as a substitute for marble in buildings, 
and also in the manufacture of plaster of Paris and Portland cement. 

Rock Salt is widely distributed, and some of its deposits are of 
great thickness. It is used as the basis of many chemical manu- 
facturing processes, e.g. making washing soda, soda ash, and hydro- 
chloric acid; also in preserving meats and fish. 

Mineral Resources of the United States. — The following table 
shows that we are extracting from the lithosphere material valued 
at about $7,000,000 every working day of the year: 

SUMMARY FOR 1907. 

Metallic. 

Pig Iron $529,958,000 

Copper 173,799,300 

Gold 90,435,700 

Silver 37,299,700 

All other metals 71,531,305 

$903,024,005 
Non-Metallic. 

Coal $614,798,898 

Petroleum and Natural Gas 172,973,524 

Cement 85,903,831 

Building Stone 71,105,805 

Abrasives 1,680,737 

Fertilizers 10,661,987 

Clay and Sand 173,435,358' 

Lime, Slate, etc 19,885,501 

Mineral Paints 2,979,158 

Unspecified 42,840,312 

$1,166,265,191 

$2,069,289,196 



266 



PHYSIOGRAPHY 




Fig. 118. — The Effects of Erosion on a Hard, Sandy Clay 
Foot of Scott's Bluff, Neb. 

Destruction of the Bed Rock. — The great mass of the mantle 
rock impresses us with the fact that the present surface of the bed 
rock must formerly have been covered with many feet of bed rock 
that has now disappeared. But the mantle rock of the land tells 
only a small part of the story. The sedimentary rocks were all 
of them made of material from former bed rock, and the loose 
material on the ocean floor, with the exception of that found in 
the deepest places, was derived from the same source. 

To produce this great mass, a quantity of solid rock much greater 
than the sum of the masses of the mantle rock, the sedimentary 
rock, and the material on the ocean floor, must have been worn 
away; for sea water contains millions of tons of matter dissolved 
from both bed rock and mantle rock, and portions of the bed 
rock have been deposited and worn away again more than once. 

If all this material came from the land at present known, it is 
evident that the total is equivalent to the removal of thousands 
of feet of bed rock from all the land of the earth. This great 
quantity of material has not been removed with equal rapidity 
in all parts. The rocks have been worn away more rapidly where 



THE BED ROCK 



267 




they were weak or where the agents were particularly active, mak- 
ing the land uneven, developing valleys, canons, gorges and fiords; 
and leaving mountain peaks and ridges, or mesas and buttes. 

We use the general term erosion to designate the act of wearing 
away the land. The principal sub-processes of erosion are weather- 
ing, corrosion, and transportation. Weathering differs from the 



268 PHYSIOGRAPHY 

other processes in that it disintegrates rock but does not transport 
the rock waste, whereas each of the other processes disintegrates 
the rock, transports the rock waste, and deposits it elsewhere. 
Each of these processes has been discussed in other chapters, and 
we have seen that each of them wears away rock in a character- 
istic manner and forms physical features both by erosion and by 
deposition which are easily distinguished. 




Fig. i 20. — Young Mountains, Showing Slight Effects of Erosion 

The Story Recorded in ttte Bed Rock. — As one walks along the sea- 
shore he notes that the beach is strewn with shells and sea weed, and 
that many living species make their homes in the sands. Occasionally 
one sees a fish that has been washed ashore, or finds relics from the 
plant and animal life of the land, such as leaves blown by the wind or 
bones brought by some predatory animal. 

If such a beach should be consolidated the sandstone formed would 
preserve many of the modern kinds of life and from them students of 
the distant future could learn much about our forms of animals and 
plants. The great series of sedimentary rocks contains just such a 
record of the forms of plant and animal life of the past, and since each 
layer is older than all of those deposited above it, we are able to deter- 
mine the order in which the various types of life occupied the earth's 
surface. The various layers may be likened to the leaves of a book, 
each one of which bears a record of the kinds of life which dwelt upon 
the earth at a given time. 

A study of this record reveals some very interesting facts. The lowest 
layers of the sedimentary rocks rest upon igneous or metamorphic rocks 
containing few indications that there was any form of life on the earth 
while they were being formed. In the lowest layers of the sedimentary 
rocks, however, we find many shells and the remains of animals resem- 
bling in some respects the horseshoe crab, but no remains of any higher . 
form. As we examine the layers above the lowest we find that these 
simple forms are replaced by more and more complex species of the same 
families, and then fish appear. As we ascend, layers are reached con- 



THE BED ROCK 269 

taining the remains of reptiles, small at first, but finally specimens of 
gigantic size are reached. In succeeding layers reptiles decrease in 
importance and are replaced by mammals related to the elephant and 
the dog. In the upper layers the remains of modern animals are found, 
but the remains of man and his implements and works are confined al- 
most entirely to the mantle rock. This record of the rocks tells us that 
animal life began with very simple forms which gradually increased in 
complexity of structure and brain power and that by slow steps, through 
long ages, the present forms of life have been developed. 

QUESTIONS 

1. Mention six gems that belong to the quartz group of minerals. 

2. State two properties common to both quartz and feldspar. State 
two properties, either one of which would distinguish quartz from feld- 
spar. 

3. Find five illustrations in this book showing stratified rock, and 
three showing unstratified. 

4. State four properties in which calcifce differs from feldspar. 

5. State two points of resemblance and two of difference between 
sandstone and shale. 

6. Which is the more common surface rock in the United States, sedi- 
mentary, igneous or metamorphic? Which least common? 

7. What States yield coal? See Fig. 117. 

8. How could you determine whether a given specimen was sandstone 
or limestone, if you had no acid? 

q. Why is rock forming near the shore of some coral islands a pure 
limestone? 

10. How may slate be distinguished from shale? 

ir. Coarse-grained granite is now on the surface in New England; 
what does this prove concerning the elevation of the surface iD these 
localities at the time when the granite was forming? 



CHAPTER XIX 

THE GROUND WATER 

What Becomes of the Rain. — We all know that rain either dries 
up, or runs off in streams, or sinks into the ground. In Arizona, 
for example, where the air is dry and warm, a large percentage is 
evaporated. On steep hillsides a large percentage becomes run-off 
into streams. The streams receive a large portion of spring rains 
because the air, being cold and moist, evaporates but little, and 
the ground, being frozen, cannot absorb it. But when rain falls 
on loose soil, especially where it is level or gently sloping, a large 
percentage enters the ground and is called ground water. 




Sheet of water - 



— 2__ — -^-^^^^ Impermeable layer - 

Fig. 121. — Water Table with its Hills and Valleys 

Importance of Ground Water. — This water that enters the 
ground supplies our wells and springs, keeps our rivers from run- 
ning dry between rains, and deposits valuable ores and metals; 
but its most important work is to dissolve rocks and minerals and 
to furnish liquid food to plants, thus making agriculture possible. 

Water Table. — Surface water tends to sink into the ground 
until it reaches the saturated portion. This saturated portion 
extends deep into the earth, perhaps into the heated interior, for 



THE GROUND WATER 



271 



steam forms a considerable part of the product of volcanoes. 
The ivater table is the upper surface of the saturated portion of 
the soil and rocks. The level of water in wells indicates the 
height of the water table. In general, the water table has its 
hills and valleys corresponding to those of the surface, but with 




Fig. 122. — Water Table in Relation to Lake, Marshes, and 

Springs 



Sprwj 



less difference in elevation, for it is nearer the surface in valleys 
than under hills. In fact, it frequently comes to the surface in 
valleys, forming springs, swamps, and lakes. The beds of per- 
manent streams reach below the dry-weather level of the water 
table; streams whose beds are above the water table tend to sink 
into the ground and disappear. The same is true of lakes and 
swamps. 




Fig. 123.— Water Table Lowered by Ditch 

The water table rises after a heavy rainfall and sinks during a 
dry season. Wherever its surface is inclined there is a constant 
movement of the water toward the lower portions, causing a 
renewal or change of water, not only in springs, but also in wells. 

The water in a well sunk in coarse sand and gravel, in Madison, 
Wisconsin, stands 52 feet above that of a lake 1,250 feet distant, show- 
ing a slope of 1 foot in 24, while in Long Island a slope of 1 in 440 has 
been found. When water is pumped rapidly from a well, the slope of 
the surrounding water table becomes steep, and there is a more rapid 
movement of water toward the well. 



272 



PHYSIOGRAPHY 



Wells. — Wells are holes dug or bored for the purpose of supply- 
ing water. To be permanent they must sink below the dry- 
weather level of the water table. They should be so situated and 
so protected that surface filth and impurities cannot drain into 
them. Clear, tasteless, odorless water may yet be dangerous. 



■ Well fails 1/1 dru . 

• Never - foi/ing wr/l 

' hie/ wco/her spring 




Perenniol ^pr/'ng. 



Fig. 124. — Water Table, Wells, and Springs 

When water comes from great depths it is well filtered and freer 
from surface impurities, such as typhoid fever germs. 

Many small communities depend upon wells for their water 
supply, but most large cities have abandoned them, using instead 
the water from lakes and rivers. 



Well drained by pumping 
of deep well. 



Deep welljrom which alorge 
volume of ryoter is pumped. 




Fig. 125. — Effects of Pumping on Water Table and Well 

Artesian Wells. — In ordinary wells the water level coincides 
with the local water table; but this is not the case in artesian wells. 
Artesian wells are wells in which the water level is independent of the 
local water table and dependent upon pressure transmitted from some 



THE GROUND WATER 



273 



distant water table. The first artesian wells, in Artois, near Paris, 
overflowed, forming what are called flowing wells. 

The conditions necessary for an artesian well are an impervious 
layer overlying a porous layer, such as sand or gravel, that receives 
water from some level higher than that of the bottom of the well. 
There may be an underlying impervious layer, but this is not neces- 
sary. The water in the well tends to rise to the level of the water 
in the porous layer, and it may overflow. 




Fig. 126. — Diagram of an Artesian Well 



These necessary conditions are frequently found on plains slop- 
ing gently from higher land, such as the Great Plains sloping 
eastward from the Rocky Mountains, and in the Atlantic and 
Gulf plains in southeastern United States. At Atlantic City, 
artesian wells 800 feet deep furnish water that fell miles away on 
the mainland, and flowed under the salt water of the marshes off 
the coast of New Jersey. Some of the water in the artesian wells 
of the eastern shore of Maryland has passed beneath Chesapeake 
Bay; in Calais, northern France, water is drunk that fell as rain 
on the English side of the Strait of Dover. 

To Illustrate the Action of an Artesian Well. — Take a basin, represent- 
ing the lower impervious layer, and partly fill it with a mixture of sand 
and water. Force down into the mixture a second (tin) basin with a 
hole in it. How high will the water spout? 



274 PHYSIOGRAPHY 

Springs. — When ground water forms a small stream, it tends to 
follow joints and cracks and the tops of impervious layers. When 
the stream comes naturally to the surface it is a spring. Some- 
times there is a line of springs along a hillside, just above an 
impervious layer. A spring rising through sand may form a 
dangerous or troublesome quicksand. In regions with plentiful 
rainfall springs are numerous. A permanent spring is a valuable 




Fig. 127. — Spring from Cave Formed by Stream Following a Fault 

asset on any farm. In arid regions they are especially valuable, 
for around them is found the only productive land. Oases in the 
desert owe their fertility to springs and wells. 

Hot Springs. — Hot springs are formed when their waters come from 
great depths or from near hot lava. Such springs are generally mineral 
springs also, for warm water is a better solvent than cold water. Many 
mineral springs have medicinal properties and become health and pleasure 
resorts. Well-known examples are Saratoga Springs, New York; White 
Sulphur Springs, Virginia; Hot Springs, Arkansas; Bath, England; Vichy, 
France, and Karlsbad, Bohemia. 

Geysers. — Geysers are explosive springs of boiling water whose eruptions 
occur at rather regular intervals. They are found in the volcanic regions 



THE GROUND WATER 



275 



of Yellowstone Park, Iceland, and New Zealand. The water is boiling 
hot because it comes in contact with highly heated rock. The boiling 
point is above 212 Fahrenheit, owing to the pressure of the column of 
water above. The irregularities of the tube probably make convectional 
interchanges of water difficult and slow. When, in spite of the great 
pressure, the boiling point is reached and steam is formed below, causing 
some of the surface waters to overflow, the diminished pressure lowers 
the boiling point of the water and enables great quantities much above 
its boiling temperature to flash into steam. The explosion of the steam 
expels the overlying water with great force, often sending the column 200 
feet above the crater. The operation is repeated at intervals. Beauti- 
ful white deposits are formed around geysers. 




Fig. 128. — Deposits from Hot Springs, Yellowstone National Park 

Destructive Action of Ground Water. — Ground water absorbs 
carbon dioxide from the air and acids from decaying plants and 
animals so that it becomes a weak acid. When it passes through 
the rocks it tends to weather and to dissolve it, weathering it to 
mantle rock. By weathering the mantle rock it assists in the 
formation of soil. This is the most important destructive work 
of ground water. A more spectacular work is seen as it passes 
through limestone. It slowly dissolves the limestone and forms 
passages, and in some regions large caves. 



276 



PHYSIOGRAPHY 



Mammoth Cave. — The largest known cave in the world is Mammoth 
Cave, Kentucky. It is over nine miles from entrance to farthest recess. 
It has a network of numerous galleries and passages which cross and 
recross one another, with a total length of over two hundred miles. It 
has its own rivers and lakes, in which are found sightless crayfish. 
Countless bats cling to its walls. Its blind rats have very long sensitive 




Fig. i2g. — Diagram Showing Cave and Natural Bridge 
AA, Layers of limestone; BB, sink holes; CC, vertical shafts or domes; and DD, hori- 
zontal galleries with stalactites, stalagmites, and pillars. 

whiskers for feelers. The many blind forms of animal life are probably 
the descendants of normal animals that entered the cave long ago. 
Skeletons of men have been found there. 

The best preserved skeletons of prehistoric man and the best samples 
of his hand work have been obtained from the limestone caves of France, 
Belgium, and Spain. 

In limestone regions the surface streams sometimes fall into 
sink holes, that lead to underground passages. Where sink holes 
are numerous there may be no surface streams, as in the Karst 
region east of the Adriatic Sea. Occasionally, too, portions of the 
roof of a cave fall in, leaving a portion that forms a natural bridge. 
Natural bridges are also formed in other ways. 

Underground streams do the same kinds of work that the sur- 
face streams do. In addition to this, some of them, where they 
are closely confined, wear away the roof above them. 

Constructive Work of Ground Water. — When water is evapo- 
rated it deposits its dissolved mineral matter. An example of 
this is the scale on the inside of a tea-kettle, or of a steam boiler, 
in which hard water (from limestone regions) has been boiled. In 
regions of limited rainfall the evaporation of the ground water 
leaves on the surface deposits that render the soil unfit for agricul- 
ture. In certain portions of New Mexico, where irrigation has 
been introduced, these alkali deposits are removed by flooding. 



THE GROUND WATER 



277 




Fig. 130. -Natural Bridge in Sandstone 
Near Buffalo Gap, South Dakota. 



278 



PHYSIOGRAPHY 



In some regions the matter deposited underground acts as a 
cement, consolidating the mantle rock. Sometimes crevices in 
the rocks are filled with minerals, from solutions, forming veins. 
They are to be distinguished from dikes, in which the crevices 
are filled with what was once molten matter. Hot water is, as 
a rule, a better solvent than cold water. Water from deep in the 




Fig. 131. — The Side of a "Sink" and Entrance to a Cave in West Virginia 
The stream that enters here reappears three-quarters of a mile away. 

ground is hot. When such water approaches the surface it cools 
and deposits are formed. Many veins of ore originated in this way. 

Where water containing dissolved limestone slowly drips from 
the ceiling of a cave, the evaporation of the water and the loss of 
the dissolved carbon dioxide cause the deposition of limestone, 
both as stalactites, hanging like icicles of stone from the ceiling, 
and as stalagmites, on the floor of the cave. A stalagmite may unite 
with a stalactite to form a pillar. 



THE GROUND WATER 279 

Precipitation. — Sulphur springs are very common in some regions. 
If the water from such a spring should mingle with water containing 
dissolved compounds of silver, gold, copper, lead, or zinc, these metals 
would be precipitated in combination with the sulphur as sulphides, 
and would fill the channels through which the mingled streams 
passed. Other deposits are due to the diminished pressure as the water 
approaches the surface. This permits the escape of some of the dissolved 
gases which, like carbon dioxide, are necessary to hold the mineral in 
solution. Still other deposits from solution are caused by certain minute 
plants called alga\ A compact limestone is deposited about hot springs 
in Yellowstone National Park. The white deposits about geysers are 
believed to be due to the same action. 

Petrifaction. — Petrifaction consists in the slow solution of certain 
organic substances in the earth, and the replacement of the dissolved 
molecules with deposits of mineral. Many fossils, originally in lime- 
stone, have been exactly reproduced in quartz or in iron pyrites. 

Capillary Water.— If the corner of a lump of sugar touches a 
liquid, like coffee or water, the liquid quickly spreads through the 
whole lump. In a similar way water spreads through soil, cover- 
ing each soil particle with a thin film called capillary water. This 
capillary water makes the soil moist, and even in a dry season 
supplies water to the roots of plants above the water table. It 
also transports water from the water table to the surface, where 
it is evaporated. 

Capillary Action. — As the result of the attraction of water parti- 
cles for one another and of glass for water particles, water is 
attracted a short distance up the side of a glass of w r ater. Water 
rises about Y% of an inch in a glass tube with a bore of T 1 „ of an 
inch; but it rises 6 inches when the bore is tott of an inch. 
This shows that water rises higher in tubes of smaller bore. 

In the soil, the small spaces practically form slender tubes or 
passages through which water moves. If the passages are dimin- 
ished in size, capillary action is facilitated, and the water tends to 
collect there. Advantage is sometimes taken of this when a per- 
son steps upon the place where seeds have just, been planted. 
The weight of the body presses the soil around the seeds, capillary 
action is assisted, water collects, and the seed germinates more 
quickly. 



280 PHYSIOGRAPHY 

On the other hand, if the spaces are increased, as for example 
when soil is plowed or cultivated, capillary action is interfered 
with. The ground water does not rise so rapidly and evaporation 
is retarded, leaving more water in the ground to nourish the crop. 
The loosened soil particles resulting from cultivation constitute 
what is called a dust mulch. 

Although the loss of ground water through evaporation is very 
great, yet in the eastern part of the United States this is in large 
part counterbalanced by plentiful rainfall. But even here many 
a farmer has learned by experience the \alue of cultivating his 
corn in a dry season, though there are no weeds to be killed. 

Dry Farming. — The value of the dust mulch is greatest where 
rainfall is scanty. In the Great Plains, just east of the Rockies, 
the rainfall is under 20 inches, not enough for agriculture under 
old methods. But dust mulching after every rain has made parts 
of this region, formerly called the Great American Desert, blossom 
as the rose. 

In portions of both the Great Plains and the Plateau Region 
the rainfall is too scanty, even with careful dust mulching, to raise 
a good crop every year. So the farmers not only cultivate after 
every rain, but they also refrain for a season from planting any 
crop. This cultivating without planting (called summer fallow) 
stores the rainfall until there is sufficient ground water for a 
bountiful crop. 

The practice of alternate cropping and summer fallowing is, according 
to the Year Book of the Department of Agriculture for 1907, a common 
one in the semi-arid region. But the practice of allowing the soil to 
remain bare during the entire season is questionable, for it must neces- 
sarily result in an almost complete destruction of the organic matter in 
the soil. A much better practice is to raise some kind of leguminous 
crop which can be turned under while there is still a sufficient amount of 
moisture in the plants and in the soil to cause rapid decomposition. 

Not satisfied with preventing evaporation and conserving the 
rain of two or more seasons, some farmers cause the ground water 
to collect near the roots of the plants, where it will do the most 



THE GROUND WATER 28 1 

good. This is done by subsurface packing. The sub-surface 
packer forces the soil particles a little below the surface nearer 
together. This facilitates capillary action so that the ground 
water tends to rise and to collect in the packed earth. The dust 
mulch is used to prevent its escape by evaporation. 

Dry farming is the method by which crops are raised in regions 
of deficient rainfall. It applies the principles of dust mulching, 




SHADED AREAS V^~ 
HAVE ENOUGH < 
BAIS FOR FARMING 



Fig. 132. — Dry Farming is Successful in the Areas Unshaded in the 2.Iap 



summer fallowing, sub-surface packing, and the selection of drought- 
resisting crops. 

Forests. — Regions with sufficient rainfall, well distributed throughout 
the growing season, are usually forested. A forest is a growth of trees 
sufficiently dense to form a fairly unbroken canopy of trees. Most of 
our trees are deciduous, that is, they shed their leaves in winter. The 
accumulation and decay of these leaves give to a forest a peculiarly rich 
soil of its own making and covered with leaves that tend to prevent 
evaporation of soil moisture. 

Value of Forests. — Trees furnish us all of our wood and lumber. They 
furnish the fuel of the country, most of it directly, but part of it indirectly 
through coal. They furnish food — fruits, nuts, maple sugar, etc. They 
furnish many valuable raw materials used in manufacturing — paper, 
tanning materials, wood alcohol, tar, pitch, turpentine, resin, fibres. 
They improve the climate by setting oxygen free in the process of making 
starch; as windbreaks they check the destructiveness of the winds; the 



282 



PHYSIOGRAPHY 



evaporation of their moisture and their shade cool the air, making it 
moister and subject to changes that are less and slower than in the 
neighboring open country. They affect drainage; by retarding the 
melting of the snow they prevent spring freshets; the leaves and loose 
soil retain the rain that would otherwise flow off rapidly in floods ; the 
water so retained is doled out, so that the streams and their water power 
are maintained during dry weather. 

The removal of forests causes floods that destroy property and life, 
remove the humus and fertile soils, and fill the streams with soil and rock 
waste. The coarse rock waste of the freshets is spread over the low 




LUMBER REGIONS \ r . 

| ■ | Heavily Timbered 
^~1 Moderately Timbered 




Fig. 133. — The Lumber Regions 



ground, thus destroying what has been valuable farming land. The water 
power is lessened or destroyed during dry seasons and may be too great 
to be utilized during floods. 

"Forestry is the preservation of forests by wise use." — Roosevelt. 
Our 300,000 square miles of national forest are protected by the United 
States Forest Service, which administers public property estimated to be 
worth over $2,000,000,000. The Service aims to diffuse information, to 
prevent the spread of fires, to destroy injurious insects and fungi, to 
restrict cattle grazing in forests to certain seasons, to insist that each 
tree cut be replaced by another of the same kind. 

Forests cover 550,000,000 acres, about one-fourth of the United States, 
and Forestry is becoming more and more important, for a timber famine, 
especially in the hard woods, is upon us. Forestry is very promising as a 
profession. 



THE GROUND WATER 283 

QUESTIONS 

1. Would a rainy month in the spring cause floods of the same size as 
an equally rainy month in the fall? Why? 

2. Why is the water table not level? Would it be more nearly level 
in sand, in gravel, or ordinary soil? Why? 

3. Illustrate by means of a diagram properly labeled the position of 
the water table and a spring, a permanent stream, a temporary stream, 
a marsh, and a lake. 

4. With the vertical scale 1 inch equals 100 feet, draw a diagram for 
an artesian well that sends water 50 feet into the air. Name and label 
the parts. 

5. Show the proper relative positions of a house, barn, well, and out- 
buildings on a side hill. Give your reasons. 

6. Why do cities generally depend for their water supply upon lakes 
or rivers instead of upon wells? 

7. Which would be more apt to be brackish or mineral, water from 
springs in a desert, or from springs in a well-watered region? Which 
might seem better? Why? 

8. Illustrate relations of stalactite, stalagmite, and pillar. 

9. Fill three flower pots of the same size with the same amount of soils 
of uniform texture. Cover one with straw, another with dust mulch, and 
"puddle" the top of the other (that is, wet it so that when it dries it 
will cake as a dried mud pie). When all are prepared weigh them. Set 
each in a saucer with a weighed amount of water. As the water is 
absorbed from the bottom receptacle, keep filling it up, being careful to 
weigh or to measure carefully the amounts supplied. Compare the 
amounts absorbed and state your generalization therefrom. 

10. How would living in a cave without light affect the various senses 
of animals compelled to live there for many generations? 

1 1 . Why are regions which are believed to be the roots of worn down 
mountains so frequently rich in ores? 

12. How may geysers choke themselves until they no longer erupt? 

13. How will the evaporation of water, furnished by irrigation, affect 
the amount of soluble plant food in the soil below the surface and at 
the surface? 

14. "What's the use of cultivating corn when there are no weeds in it?" 



CHAPTER XX 
THE WORK OF RIVERS 

The rivers of the United States furnish power to great manufac- 
turing industries, supply water to cities, and transport every year 
hundreds of thousands of people and millions of tons of merchan- 
dise and farm products. They have figured in the history of our 
country from the beginning, both in its peaceful settlement and 
development and in war. The Hudson River was of vital strategic 
importance during the Revolution, and the Mississippi and the 
Tennessee during the Civil War. 

In the economy of nature streams have many functions, the most 
obvious of which is the removal of the surplus rainfall. But while 
doing this, all streams, from the largest river to the tiniest brook, 
are slowly wearing down the land. 

The Formation and Development of a Gully. — To study the 
principal phenomena of the work of a river in wearing down the 
land, it is not necessary to go farther than to the nearest bank of 
soft earth and to note what happens during and immediately 
after a heavy rain. The water collects in a stream and flows over 
the edge of the bank and down its slope, quickly forming a minia- 
ture valley or gully. 

If the stream is swift and the bank soft and steep, the valley 
deepens rapidly. The steepest place is at the edge of the bank, 
and here the stream cuts into the earth and the valley lengthens 
headwards. During a heavy rain the sides of the gully are washed 
in and the valley widens rapidly. 

The materials washed out of the gully are, in part, deposited at 
the foot of the bank, where the slope becomes gentler. Here the 
stream is building up instead of cutting down. When the rain 
slackens it may be possible to see the stream diminish in size after 



THE WORK OF RIVERS 285 

a time, and the stream-deposits extend up the gully, partially 
filling it and making the miniature valley flat-bottomed. 

Sometimes for several days a small stream, fed by some tem- 
porary spring, will flow down this flat-bottomed valley in a wind- 



■j 3 -^ 


\ ""**-• - JW- 



Fig. 134. — Gully and Alluvial Cone, Formed in a Single Shower 
Near Baraboo, Wisconsin. Note coarse stones in gully. (Eliot Black welder.) 

ing course, cutting into its outer or concave bank at every curve. 
Here the banks are generally higher and steeper and the stream 
deeper. The convex inner bank is lower and more gently sloping, 
with deposits in front. 

If the gully is examined after the stream has disappeared, it 
will be seen that in places the slope is too steep for any deposits 
to be made, but where deposits are made in steep places coarse 
materials predominate, whereas deposits made on gentler slopes 
are not so coarse. The result is an assorting of 'deposited mate- 
rials. This is noticeable in the deposits at the foot of the slope. 

Sometimes a stone in the path of the stream causes a fall to 
form. Just below the fall the water wears out a hole deeper than 



286 



PHYSIOGRAPHY 



the average of the other portions of the stream. A short distance 

below the fall there is frequently a deposit in the bed of the stream. 

Most of these phenomena of the gully can be seen without much 




Fig. 135. — Man Measuring Stream Flow 

difficulty along every stream, in some portion of its course. Gully 
and stream work may be summarized in a sentence: Streams drain 
away the surplus rainfall, and in so doing wear down the land, and 
transport, comminute, and finally deposit the waste so formed. 

DRAINAGE 

Drainage. — Our rivers remove the equivalent of about 10 inches 
of rainfall per year from the whole United States. The Mississippi 
annually carries to the sea about one-ninth of the rainfall of the 
whole country, an estimated total of 44.7 cubic miles. This is 
enough to make a lake the size of the State of Illinois, and four feet 
deep. 



THE WORK OF RIVERS 



287 




The economic value of this 
water is such that the National 
Government is seeking to de- 
termine the best methods of 
storing and conserving it for 
irrigation and power pur- 
poses. Government officials 
have estimated that the 
streams of the Southern 
Appalachians alone have 
1,400,000 undeveloped horse- 
power, worth, at $20 each, 
$28,000,000 per year. 

By saving and doling out 
in dry seasons the water of 
floods, mills can be kept 
going that otherwise might 
be compelled to close. If 
water power, often called 
white coal, could be used 
instead of coal for power purposes, our diminishing coal deposits 
would be conserved. 

CARRASION 

Corrasion. — Streams wear away their beds and banks, forming 
most of the valleys of the world and obtaining materials that are 
eventually deposited in the sea. Stream corrasion is the wearing 
away of rocks by running water. Part of this is due to the solvent 
action of water. Clear water alone is a poor corrading agent. 
Just as paper by itself is a poor abrading agent but becomes an 
efficient one when covered with sand as sand-paper, so water 
supplied with sand and rock fragments as tools becomes an effi- 
cient corrading agent. A load of sand thrown into' the clear water 
of the Niagara River from the bridge just above the American 
Falls soon scours away the moss that the water alone is not able 
to remove from the rocks in the bed of the river. 



Fig. 136. — Some Instruments Used in 
Measuring Stream Flow 



288 



PHYSIOGRAPHY 



The rate of corrasion depends upon the resistance of the materials 
forming the stream bed, the volume and velocity of the water, and 
the kind and amount of material transported by the stream and used 
as corrading tools. In 1906 the Colorado River, which supplied 
water to an irrigating ditch leading to the Imperial Valley region 
of southern California, got beyond control. In the weak deposits 



Bee.-ft. 


JAN. 
I0 2O 


FEB. 
10 20 


MAR. 
10.20 


APR. 
10 20 


MAY 
10 20 


JUNE 
tO 20 


JULY 
10 20 


AUG. 
10 20 


SEPT. 
10 20 


OCT. 
10 20 


NOV. 
10 20 


DEC. 
K^0 2O 


































oH 






































6,000 
6,000 
4,000 
3,000 
2,000 
J, 000 

















































































































































































































































































































































































































































































































C 



Fig. 137. — Stream Flow in Region of Little Rainfall 

Discharge of West Gallatin River at Salesville, Montana, for 1899. 

(U. S. Geological Survey.) 

of the old delta that the Colorado had built across the Gulf of 
California, it soon changed a small irrigating ditch to a channel 
large enough to receive half of the river. In the depression that 
had once been the head of the Gulf, the Salton Sea was formed, a 
lake larger at one time than Lake Champlain. A fall over ninety 
feet high and 1,500 feet wide cut back toward the Colorado at 
the rate of half a mile a day. After threatening the destruction 
of $100,000,000 worth of property, the river was, with much diffi- 
culty and at great expense, finally brought under control. 

(a) Downward Corrasion. — Every stream is in some part of its 
course corrading its bed, thus forming or deepening its valley. The 



THE WORK OF RIVERS 



289 



best example in the world of the work of downward corrasion is 
the Grand Canon of the Colorado, 300 miles long and in places 
over a mile deep, all cut out little by little by the Colorado River. 




Fig. 138. — Gorge Coreaded in Shales. Watkins Glen, N. Y. 
(U. S. Geological Survey.) 



The deepening of its valley is the first work of a river, for the 
water brings its corrading tools into direct contact with its bed 
and wears the bed away. The stream tends to cut down verti- 
cally and to form narrow, deep valleys with precipitous sides, 



290 



PHYSIOGRAPHY 



called gorges or canons. An overhanging side may be the result 
either of curving and consequent undercutting by the stream, or 
it may be due to the rock structure. 

The widening of a river valley is largely the result of weathering. 
The side walls are disintegrated and the particles of rock fall and 
are washed into the stream and carried away. If the materials 
of the sides of the valley are sand or gravel, the sides cannot be 
very steep, otherwise the materials would roll down the slope. 




Fig. 139. — Diagram of Gradual Widening of Valley 



The rate of widening determines the shape of the valley as seen in 
cross-section. Torrents may retain for a time valleys with almost 
vertical sides, such as is seen at AAA in Fig. 139; but as down- 
cutting becomes less rapid, and the weathering agents widen the 
valley, it becomes V-shaped in cross-section, as B B B. As the 
valley widens more and more, it assumes in turn shapes more 
like C C C, D D D, and E E E, becoming wider and wider, until 
its sides have a very gentle slope. 

When the valley is cut through rocks of different hardness, the 
weaker rocks are worn away more rapidly than the more resistant, 
which may stand out as cliffs, their upper surfaces forming rock 
terraces, as in Fig. 140. 

A river valley becomes longer, just as a gully develops, by corra- 
sion at the very head of its valley. This headward corrasion 



THE WORK OF RIVERS 



291 




Fig. 140. — Rock Terraces 



increases the length and decreases the steepness of the stream, as 
is shown in the series of profiles in Fig. 141. A river profile is a 
line showing the slope of the surface of a river from its source to its 
mouth, according to definite vertical and horizontal scales. The 



B 



M 



Fig. 141. — Progressive Headward Corrasion 



profile A E M shows a steep slope near the source, becoming gen- 
tler down stream. Profiles BFEM,CGFEM, andDGFEM 
show progressive headward corrasion. 

When a stream encounters in its bed rocks of different hardness, 
it wears away the weaker rocks more rapidly, producing at the 
hard rocks falls and rapids. At the foot of the falls the water 
swirls stones and bowlders and tends to wear there a depression 
called a pothole. 

Most land surfaces are so uneven that the course to be taken 



292 



PHYSIOGRAPHY 



by rains falling on them is predetermined. In such regions it is 
easy to locate the water parting or divide. A divide is the line 
separating two adjacent river basins. Sometimes the area between 
two streams is so nearly level that the course rainfall will take is 
doubtful. In such a region divides are not well marked. But no 





Fig. 142. — Divides and Streams 
in Austrian Alps. (Hachure Map.) 



Fig. 143. — Divides in Preceding Region 



matter how level the region, with the headward advance of streams 
divides must eventually be developed between principal streams, 
with subdivides between tributaries. 




BASE 



LEVEL 



Fig. 144. — Divides, Valley, and Streams 

Divide A between streams X and Y; adjusted divide S between streams Y and Z, shifting 

from 1 to 5, disappears at 6, causing 2 to be captured by a tributary of Y. 



THE WORK OF RIVERS 



293 



If one stream is more actively deepening its valley than a neigh- 
boring stream, the divide between them shifts toward the weaker 
stream, as divide S in Fig. 144. But when two streams reach 
the point of lowering their basins at the same rate, the divide 
between them is adjusted, and instead of shifting sinks vertically 
as the region is worn down, as divide A in Fig. 144. 

Stream Capture. — The effects of shifting of divides are seen in 
the Shenandoah River Valley, a region of rocks less resistant than 
those of the Blue Ridge to the east of it. The master stream of 




Fig. 145. — Stream Arrangement in West Virginia and North Dakota 
Three stages in the capture of Beaver Dam Creek, B, by the Shenandoah. S. 

the Shenandoah, the Potomac, cuts through the Blue Ridge at 
Harper's Ferry, forming a water gap. Beaver Dam Creek formerly 
had its course west of the Blue Ridge, as is shown in Fig. 145. 
It then flowed through the mountain ridge at Snicker's Gap, a 
battleground of the Civil War. 

The Potomac, being larger, corraded its gap deeper than did 
the Beaver Dam; consequently the Shenandoah, a tributary of 
the Potomac, was able to corrade more deeply ifi the weak rocks 
of its valley than the Beaver Dam through the resistant rocks of 
the Blue Ridge. The divide between the two shifted nearer and 
nearer to the Beaver Dam, until finally the Upper Beaver Dam 



2Q4 PHYSIOGRAPHY 

was captured and became a tributary of the Shenandoah. The 
present Beaver Dam is a beheaded stream; and the abandoned 
water gap is a wind gap. From Snicker's Gap to the Shenandoah 
the stream is reversed in direction. This action by the Shenandoah 
illustrates one method of stream capture or river piracy. 

Tributaries. — When several streams flowing down the same gen- 
eral slope unite, they form a tree-like system of drainage in which 




Fig. 146. — Meandering Stream in a Narrow Flood Plain 
Canon del Muerte, viewed from Mummy Cave. 

is illustrated the general rule that tributaries join their master 
stream at an acute angle pointing down stream. 

Tributaries, though smaller, are generally steeper and swifter 
than their master stream, and may be corrading their beds more 
rapidly; but it is clear that they cannot corrade deeper than their 
master stream where they join. This tendency of tributaries to 
corrade their beds at such a rate as to join their master stream at 
its grade, or level, is known as Playf air's Law. 



THE WORK OF RIVERS 



295 



Stream capture, as we have seen, may result in causing tribu- 
taries to join the capturing stream at a right angle, or even at an 
acute angle pointing up stream. The Potomac and the Delaware 
have tributaries joining them at right angles; and Schoharie 
Creek, N. Y., and the Maumee River, in Ohio, have tributaries 
that join in at acute angles pointing up stream. 

(b) Lateral Corrasion. — Every stream is cutting into its outer 
bank at every curve, because water obeys the law of motion that 




B Vine'' — C 

TILLING %nr 



CUTTING 



Fig. 147. — Development of a Meander 

bodies in motion continue in motion in a straight line and at a 
uniform rate unless acted upon by some outside force. 

If any curve in any stream is examined, it is found that the main 
channel and the swiftest current approach the outside of the curve. 
Let this be indicated in Fig. 147 by a dotted line, and let an arrow 
indicate the direction of flow. Just above and below the curve, the 
main channel, at A and D, is in the middle of the stream. But 
between B and C the water (obeying the law of motion) ap- 
proaches B, and the swiftest current and deepest water are found 
nearer B than C. The swift current cuts into the bank at B and 
tends to undermine it. Soon a portion of the bank at B falls into 
the stream, and is washed away. This operation continues, and 
the channel moves more and more toward B. 



296 PHYSIOGRAPHY 

On the opposite side, at C, on the inside of the curve where the 
bank is convex, the velocity of the water is less and the stream 
deposits a part of its load, building out this side. 

This cutting and filling action continues at every curve, and the 
stream tends to develop a series of winding curves called meanders. 

This tendency to meander is best seen where the materials 




Fig. 148. — Meandering Streams, Laramie Creek, Wyoming 
Notice nearest curve bank, high on our right, low on left. 

composing the banks of the stream are most easily corraded. These 
cnoditions are found in the low " bottom " lands near streams 
that the water overflows when the streams are in flood. These 
lands, called flood plains, are composed of materials that the 
stream has brought there and deposited from its muddy waters. 






Fig. 149. — How Cut-off and Oxbow Lakes are Formed 



THE WORK OF RIVERS 



297 



A river may become so curved, especially where it is meander- 
ing over a large flood plain, that two curves approach each other. 
At some flood stage the water cuts through the intervening nar- 




Fig. 150. — Oxbow Lakes, Sand Deposits, and Main Channel of Portion of the 
Mississippi River 

row neck of land and forms a cut-off, as at X in Fig. 147. For 
a time the water flows through both channels; but the new is 
shorter and the current through it consequently swifter, so that 
it rapidly increases in size until all of the water passes through it. 
In this way streams tend to straighten themselves. Fig. 149. 



298 PHYSIOGRAPHY 

The ends of the old channel are almost at right angles to the 
new channel; water entering the old is checked in velocity and 
deposits materials that slowly close the entrances to the aban- 
doned channel, changing it to an oxbow or crescent lake. 

The crescent lakes of large rivers are arcs of larger circles than 
are the crescent lakes of smaller streams. There is a relation 
between the size of a stream and the size of the curves it makes 
on its flood plain. The curves of the lower Mississippi are arcs 
of circles of approximately five to ten miles in diameter. A line 
along the outside of the curves on the east side of the Mississippi 
is about fifteen miles from the corresponding line on the west 
side, making the meander belt or meander zone of the Mississippi 
about fifteen miles wide. Meanders move down stream. 

TRANSPORTATION 

Transportation. — A glass of water dipped from any muddy 
stream will become clear after standing for a longer or shorter 
period, and layers of sediment will form on the bottom of the 
glass. This solid material distributed through the water, in spite 
of its greater density, is said to be carried in suspension. 

The particles have been obtained by corrasion of the bed or of 
the banks, or may have been washed into the stream. It is pos- 
sible that the particles may travel to the mouth of the stream 
without a stop, but under ordinary conditions they settle to the 
bottom when the current slackens, to be picked up again and 
carried farther when the current is again increased. 

The finest particles of rock waste are heavier than water, and 
would settle in water at rest or in water moving in lines parallel 
to the surface. But the irregularities in the stream beds are con- 
tinually sending numerous currents upward, thus counteracting 
the tendency of the particles to settle. In comparison with a 
large particle a small particle has a larger area, and it must there- 
fore set a relatively larger mass of water in motion in order to 
sink. 

The transporting power of a stream depends mainly on the 
volume and the velocity of the stream. A stone weighs less in 



THE WORK OF RIVERS 299 

water to the extent of the weight of water it displaces; this causes 
most common rocks to lose about one-third of their weight in 
water. The transporting power of streams increases as the sixth 
power of the velocity. For example, if the velocity is trebled, the 
transporting power, instead of being trebled, is increased 729 
times, that is, 3X3X3X3X3X3- Torrents, in their steep upper 
courses, are able to roll along bowlders of many tons weight. 
A change in velocity, even if slight, makes a great change in the 
amount of sediment that may be transported. 

In swift streams much rock waste too coarse to be held in 
suspension is rolled along the bottom, and sometimes in mountain 
torrents the collisions between the stones thus moved produce 
a loud and almost continuous noise. In the lower portion of rivers 
the amount of sand and small pebbles that is rolled along the 
bottom is probably a very important part of the total amount 
of rock waste transported. 

Work in many streams is done at flood time only. During the 
summer most streams are low and do little work. 

Water in passing over soluble substances dissolves them in 
part. In limestone regions the water becomes " hard," that is, 
with soap it does not easily form lather or suds, and when boiled 
a scale forms on the inside of the boiler or of the kettle. 

The amount of mineral matter carried in solution by streams 
varies greatly. It depends, among other things, upon the nature 
of the region over which the streams flow. It is well known that 
the water of streams in sandstone regions is softer than that of 
streams in limestone regions; that is to say, they contain a smaller 
percentage of mineral matter in solution. The total amount of 
mineral matter carried to the ocean in solution is about one-third 
as great as that carried in suspension. 

A small amount of rock waste is transported on the surface of 
streams, lodged in ice or attached to the roots of trees or to other 
floating objects, called drift. Drift tends to go toward shore 
when a stream is rising and the water surface in the middle of the 
channel is highest. But when a flood is subsiding, the water 
surface is concave and the drift tends to leave the banks and to 



300 PHYSIOGRAPHY 

seek the middle of the channel. Lumbermen take advantage of 
this in floating out their logs. 

Hilly, forested portions of the land should not be stripped of 
their forests and cultivated, because this causes the streams to 
wash the soil away. Neglect of these precautions has done much 
damage in some of the older States. The Forest Service of the 
National Government is endeavoring to avert the fate that has 
overtaken certain portions of Spain and China. 

The Mississippi River removes yearly, by rolling along its bed, 
enough waste to cover a square mile to a depth of 19 feet; in 
suspension waste enough for 241 feet more; in solution 50 feet 
more if it were all limestone — a total of 310 feet. This is enough 
waste to lower the level of the whole Mississippi River Basin at 
the rate of 1 foot in about 4,000 years. 

The Po removes enough waste to lower its whole basin at the 
rate of 1 foot in every 729 years. 

COMMINUTION OF LOAD 

Grinding and Polishing. — Streams push and roll angular frag- 
ments of rock along their beds and over one another, colliding as 
they go, until their corners are knocked off and they are rounded 
and worn smooth. In this way large angular stones become small 
pebbles, characteristically smooth and rounded; just as boys' 
marbles may be made by placing small pieces of marble in a cyl- 
inder, the rotation of which causes the pieces to wear one another 
round. 

DEPOSITION 

Deposition. — A river carrying waste tends to deposit its waste 
whenever its velocity is diminished. The velocity is diminished 
by decreasing either the slope of the bed or the depth or volume of 
the water. A slight check in velocity causes only the coarsest 
materials to come to rest. Further checking deposits materials 
not quite so coarse. By this process deposits of different sized 
materials tend to form in different places at the same time, and in 
the same place at different times. As a result, stream deposits 



THE WORK OF RIVERS 



30I 







3° 2 



PHYSIOGRAPHY 



are generally assorted according to size and stratified, that is, ar- 
ranged in layers. 

Alluvial Fans and Cones. — The effect of a sudden change of 
slope is well illustrated in Fig. 134, of the gully. The water 
loses velocity as it approaches the level land, and here the coarsest 
materials are dropped. As the velocity diminishes the particles 
deposited are smaller and smaller, gravel will be carried farther 



pp 







Fig. 152. — Alluvial Cone at Mouth of Aztec Gulch, Colorado 

than the bowlders, sand farther than the gravel, and finally clay 
farther than the sand. From the foot of the slope the deposits 
spread out in a semicircular form, made up of concentric bands of 
assorted materials called alluvial fans, if of gentle slope, but allu- 
vial cones if the slope is steep. 

Streams from mountains sometimes form fans which join later- 
ally, forming plains. Because such plains from the Sierra Nevada 
are higher than those from the Coast Ranges of southern Cali- 
fornia, the San Joaquin River lies nearer to the Coast Ranges 
than to the Sierras. For a similar reason the Po lies nearer to 
the Apennines than to the Alps. 

Sand Bars. — A decrease of slope within the bed of a stream 
causes deposition forming sand bars. These bars sometimes begin 



THE WORK OF RIVERS 



3°3 



about obstructions that check the velocity of the water. Their 
formation and growth resemble that of snowdrifts and sand 
dunes. They have a gentle slope up which sand and pebbles are 
rolled, and like dunes, they migrate. They are most noticeable 
in times of low water; during high water they may be scoured 
out, but they may form again about the same place. 

Nearly all streams, large and small, are undergoing this sconr- 
and-fill process. The scouring effect is produced artificially in 




Fig. 153. — Alluvial Cone, with Tributary and Distributary Streams 
Note contours on cone. (U. S. Geological Survey.) 

South Pass, one of the mouths of the Mississippi, by means of 
jetties, which narrow the channel. The water above the jetties 
rises. The higher head of water causes the water to flow through 
the narrower space with greater velocity, scouring out the deposits 
and preventing the formation of a bar that would impede navi- 
gation. 

Braided Streams. — Some rivers have in the dry season very 
wide beds with but small volume of water. The wide and shallow 
stream cannot then carry the load brought by its tributaries and 
deposits much of the load on its own bed, forming numerous 
interlacing channels. Such a stream is said to be braided; in dry 



304 PHYSIOGRAPHY 

seasons much of its water flows underground. The Platte River 
is an example of a braided stream. 

Deposits on Flood Plains. — Water particles move about each 
other with much less friction than they move over solids, and 
also with less friction than between water and air. This 
accounts for the fact that the deepest portion of the cross- 
section of a stream is the swiftest portion, and also for the 
further fact that in this deepest portion the velocity at the bot- 
tom is less than that near the top. 

When the water of a river spreads out in shallow sheets, as it 
does when it overflows its flood plain, the velocity on the flood 



JTuoTi. Water 




Fig. 154. — Section Across Alluvial Plain on One Side of a Large River 
Vertical scale exaggerated. 

plain is much diminished by friction, whereas the velocity in the 
channel, where the water is deep, is greater than the velocity in 
the same place at low water. This not only causes deposition on 
the flood plain, but is likely to cause corrasion in the bed else- 
where, thus increasing the available materials for deposition on 
the flood plain. The deposit is called alluvium, or silt. The 
lands subject to floods tend to build up to the level of the floods, 
and are very properly called alluvial or flood plains. Because every 
flood renews the fertility of the flood plain, we find here the most 
fertile lands. The flood plain of the Nile was the granary of the 
ancient world. When the current is swift, it may wash fertile 
soil away, or cover it with gravel and bowlders. 

When water leaves the main channel and spreads over the 
flood plain its velocity is checked the most close to the stream, 
and consequently more and coarser deposits are made here than 
farther back. This excess of sandy deposits on the flood plain 
close to the stream is called a natural levee. 



THE WORK OF RIVERS 



3°S 



In the lower Mississippi, below the mouth of the Red River, 
the slope is so gentle that sediment is deposited along the bed of 
the river, raising the level of the surface of the river. Not infre- 
quently the surface of the river is higher than the land back of 
the natural levees, and this gives the river the appearance of 



INDIANA 




KENTUCKY 



|;v:;:'*1 depositing. 
Arrows show main 
channel and eddy 



Fig. 155. — The Meandering of the Great Miama River 

In a wide alluvial plain at its junction with the Ohio River. At different periods it has 
consecutively entered the Ohio through the different mouths as indicated by the dotted 
lines. As late as 1786 it occupied the bed numbered s in the map. Most of the surround- 
ing plain is covered several times a year by water in times of Hood, sometimes to the depth 
of IS feet. The amount of sediment deposited is remarkable. 

A stone monument that marks the Ohio-Indiana state line at this point when set up was of a 
height that a man on horseback could barely reach the top; at the present time the top is 
but two feet above the surface of the plain. 

Another feature of this deposition of sediment is that the older deposits now buried to the 
depth of many feet are far better suited to agricultural requirements than the present sur- 
face sediments for the reason' that the earlier deposits come from the forest-clad hills rich 
in humus, while the present are the impoverished wastings of newly-tilled, bare fields. 



3° 6 



PHYSIOGRAPHY 



flowing along in the top of a ridge it has built across its flood 
plain. 

The slope of the flood plain, away from the river, is usually- 
steeper than the general slope of the river toward its mouth. 
As a result of this, when the Mississippi overflows its banks and 
spreads out over its flood plain,, those lands most remote from the 




Fig. 156. — Mouths of the Mississippi River 
Only the natural levee portions of the deposits appear above water. (U. S. Geological Survey.) 

river are first and most deeply drowned, in some places to a depth 
of more than thirty feet. 

As the river continues to build up its front lands, there comes 
a time when the lower swampy back lands offer a more favorable 
route for the river than its normal meandering course. The river at 
some flood stage seeks this new and more favorable route. This 
sudden change of the river is called migration, and is to be distin- 
guished from the gradual shifting of the channel called meandering, 
which is confined to the meander zone. 



THE WORK OF RIVERS 307 

From Memphis to Vicksburg the Yazoo River, on the east side 
of the flood plain, occupies an abandoned channel of the Missis- 
sippi. The Yazoo is not capable of forming meanders as large as 
those it follows. South of Vicksburg, where the Mississippi fol- 
lows the east side of its flood plain, there is a similar abandoned 
channel on the west side of the flood plain, now occupied by the 
Tensas River, which is likewise incompetent to produce the wide 
meanders of its course. 

Protection from Overflow. — Two methods are advocated for 
protecting the flood plain of the Mississippi from overflow. One 
is to maintain outlets to distribute the floods as quickly as possible. 
On the east side, just below Baton Rouge, an outlet could be main- 
tained into Lake Pontchartrain, by way of Bayou Manchac, a 
former distributary; on the west side, one through the Atcha- 
falaya, one through Bayou Plaquemine, and one through Bayou 
La Fourche. 

The other method is to build levees sufficiently high to restrain 
the highest floods. This involves the building of high levees along 
both sides of the Mississippi (except where it flows near the high 
land, where but one levee is necessary) and along all important 
tributaries and distributaries. 

Levees are built of flood plain materials, the largest being about 
40 feet high and 200 feet wide at the base. In times of danger 
the height of the levee may be temporarily raised by bags of 
earth. The levees are built alongside the river where the banks 
are convex; but where the banks are concave the levee is built 
farther back because of the cutting and caving of the banks on 
this side. 

When the water is critically high, State guards patrol the levees 
on the lookout for leaks and to prevent tampering with the levees. 
Steamboats are required to keep as far away as possible from 
the levees, lest the waves generated by them cause the levees to 
break. The greatest natural enemies of the levees are the cray- 
fish and the muskrat. 

Because the flood plain is highest near the river, small streams 



308 PHYSIOGRAPHY 

starting near the main stream flow away from it toward the back 
swamp. The Yazoo River enters the flood plain and flows along the 
back swamp region parallel to the main stream until captured by 
the migration of the Mississippi to that side of its flood plain at 
Vicksburg. In Louisiana the Atchaf alaya continues along the back 
swamps to the sea. The Red River, receiving the drainage from 
the Tensas Basin of the Mississippi flood plain to the north, is, 
more and more, sending its waters to the Gulf by way of the 
Atchafalaya. 

When a river empties into a quieter body of water, as the sea 
or a lake, its velocity is checked and waste is deposited in and 
around its mouth, forming a delta. 

The several channels into which the main stream divides in 
the delta are called distributaries. Those at the mouth of the Mis- 
sissippi, called " passes," are bordered by continuations of the 
natural levees of the flood plain. 

The river-borne waste, which consists of the finest materials, 
may be carried far to sea before being deposited, sometimes as far 
as 200 miles from shore. 

QUESTIONS 

1. On the U. S. Topographic Maps of your vicinity, study carefully 
your nearest or most interesting river or stream. Apply a string care- 
fully along its course and determine the length of the stream in miles 
and kilometers. Determine its limiting divides. Estimate the area 
of its basin in square miles and in square kilometers. Map it on manila 
paper on as large a scale as is convenient. 

2. Proceed similarly for your State, mapping the canals as well as the 
principal rivers and lakes. 

3. On a U. S. Base Map, mark the principal divides and guess at, or 
estimate, the areas of the principal drainage regions in percentages of 
the whole. 

4. After you have studied a real gully, describe how, in your opinion, 
it was formed. 

5. If the average annual output of the Mississippi River is 44.7 cubic 
miles of water, how deep a lake would this make if its area was that of 
your county? your State? 



THE WORK OF RIVERS 309 

6. Draw or trace the tributaries of the Maumee River of Ohio, Scho- 
harie Creek, New York, and of some other stream system, and account 
for the ways the tributaries join their master streams. 

7 . Give at least two reasons why some streams do not corrade their beds. 

8. With a diagram explain how meanders and oxbow lakes are formed. 

9. Draw a top view, showing tributaries, distributaries, and contours 
of an alluvial cone or fan. Draw cross section showing location of 
coarsest and finest deposits. 

10. Draw a portion of the Platte River showing a braided stream. 

11. Draw and label an ideal cross section of a flood plain, showing a 
river flowing along in its bed in the top of a ridge it has built for itself. 
The bottom of the bed is to be below sea level; the river bank full and 
higher than the back swamps, which are beginning to fill with water. 

12. Summarize in tabular form under headings, (1) kinds, (2) places, 
and (3) products, the five different ways in which streams work. 

13. On transparent paper trace the divides and subdivides in Fig. 
142. Then without consulting the map, draw in, in blue, the streams 
where you think they should be. Then compare your work with 
Figs. 142 and 143. 

14. Similarly trace divides and subdivides in the mountainous 
portion of Fig. 153. Then draw in, in blue, the streams where you 
think they should be. Compare Fig. 153. 



CHAPTER XXI 

LIFE HISTORY OF A RIVER 

Base Level. — The life work of a river, with reference to the 
region it drains, is to wear down the land and to carry it into 
the sea. Its work will never be finished until the region is worn 
down to sea level. The level of the sea is the base level below 
which the lands cannot be eroded; but we may also have local 
base levels, such as a lake or the level of a stream into which a 
tributary empties. 

Stages of Development. — It is convenient to speak of the dif- 
ferent stages of a river's development as youth, maturity, and old 
age, and to characterize the general features of a region as young, 
mature, or old. These terms are relative, and do not lend them- 
selves to expression in numbers of years. 

Youth. — A stream that is degrading and has most of its work 
before it is said to be young. Young streams are characterized 
by steep slope, rapid current, and great power to corrade their beds. 
Young streams have narrow V-shaped valleys. Lakes are character- 
istic of young rivers, disappearing before maturity. At first a 
young stream has few tributaries, especially on plains and pla- 
teaus, where the tributaries may begin as mere gullies. The 
divides are not well marked, especially on plateaus and plains, 
where large level interstream areas are found. Rapids and falls 
may be present in a young stream, but they, too, disappear before 
maturity. Falls, rapids, and lakes give a young stream a profile 
that is in places convex upwards. Young streams are usually 
clear. The upper course of all great rivers is young. 

Maturity. — A river or any part of it is said to be graded or 
mature when it has a slope just suited to its load and volume. 



LIFE HISTORY OF A RIVER 311 

It has so destroyed its falls, rapids, and lakes, and so aggraded or 
built up its too gentle slopes, that it has just the right slope to 
carry its load of waste with its volume of water. Its profile is 
called the profile of equilibrium. This perfect adjustment of slope, 
volume, and load is difficult to attain. If attained, any change in 
any one of the three factors disturbs their balance. Although no 
river is graded throughout its entire course, most rivers have 
graded portions. In maturity the divides are well defined and 
adjusted, the valleys broad, and the numerous tributaries obey 
Playfair's Law of entering their main stream at the 
level of the main stream. The river tends to meander 
^•r over flood plains, becoming wider toward the mouth. 

The profile of equilibrium is a curve, concave upward, 



SEA LEVEL 




SIMILES FROM MOUTH >• 69 MILES >■ 4-7MILES 4-2MILES 



Fig. 157. — Profile of Passaic, Showing Characteristics of Youth 
Rapids at R; lake when floods at L; falls at F. 

steeper near the source, becoming more gently sloping, and pass- 
ing imperceptibly into the base level of erosion at the mouth. 
The middle course of great rivers is in the graded or mature stage. 

Old Age. — In some rivers, and especially near the mouths of 
large rivers, the slope becomes too gentle for the stream to carry 
all its load of waste, and so a part is deposited. Old streams have 
generally wide, flat-bottomed, shallow valleys, wide flood plains over 
which the streams meander, forming oxbow lakes. Since the flood 
plain is highest near the river, streams formed on the flood plain 
are usually prevented from joining the river. The deposits tend to 
build up the stream bed and, at the mouth, to form a delta that 
extends or prolongs the flood plain, and the river breaks up into 
distributaries. The lower course of many great rivers is in the 
old-age stage. 

When all the streams of a region have reached old age, and 



312 PHYSIOGRAPHY 

when the region is worn down nearly to base level, it forms what 
is called a peneplain. A peneplain is a region that is "almost-a- 
plain," and is the last stage of an eroded mountain or plateau. 

Normal River Cycle. — Because every stream tends to pass 
through youth, maturity, and old age, these stages constitute 
the normal cycle, and their record the life history of a river. Many 
streams never complete their normal cycle because it is inter- 
rupted in some way. 

Interrupted River Cycles. — The normal cycle of river develop- 
ment may be interrupted by change of slope, resulting from de- 
pression or elevation, and by change of climate from moist to dry or 
dry to moist, or from warm to glacial or glacial to warm. The gen- 
eral effect of elevation is to lengthen the cycle, of depression to 
shorten it. 

Effects of Depression. — If depression occurs at the mouth of a 
river, the sea will enter the lower portions of the river valleys, 
drowning them and producing bays, estuaries, or fiords. Tribu- 
taries near the mouth of a river enter bays, and the master stream 
is said to be dismembered. The lower Susquehanna, with its tribu- 
taries, the James, the York, and the Potomac, when drowned and 
dismembered, becomes Chesapeake Bay, with its many branching 
bays. 

Effects of Elevation. — Elevation of a region at the mouth of a 
river lengthens the river. When two or more rivers thus length- 
ened unite, they form an engrafted river. Rivers may also be en- 
grafted by the extension of their deltas into the same bay. The 
tributaries of the Mississippi River below Cairo have been en- 
grafted upon it. 

If elevation takes place at the source, the slope is increased, and 
the river is rejuvenated. A meandering river, if rejuvenated, forms 
entrenched meanders. Rivers entrenching themselves in flood 
plains sometimes leave portions of the old flood plain persisting 
as alluvial terraces. 

If the uplift is across its course, the river may corrade down- 
ward as rapidly as the uplift is made, thus producing water gaps. 
The Green River passes through the Uinta Mountains and the 



LIFE HISTORY OF A RIVER 313 

Hudson through the Highlands. Because such rivers had their 
approximate location before the mountains were uplifted they are 
called antecedent rivers. 

Effects of Change of Climate from Moist to Arid. — In gen- 
eral the river cycle is lengthened. When the annual rainfall of a 
region diminishes the rivers become smaller and eventually cease 
flowing, except immediately after rains. Forests disappear except 
along stream courses. When forests are removed there is little to 
retain the run-off, and the streams, quickly flooded, quickly subside. 
Their dry stream beds are called gulches or wadies. The lakes by 
evaporation become salt, and generally decrease in size, finally 
becoming salinas or salt plains. When the inflowing streams bring 
much sediment, playas or mud plains may be formed. 

The region between southern Russia and Pekin, China, exhibits 
these phenomena in different stages. Increasing dryness in what 
is now the desert portions of the Chinese Empire may have caused 
the great migrations of the Asiatics which resulted in the invasion 
of Europe by the Huns and Mongols. 

Similar climatic changes have taken place in the Great Basin 
portion of the United States between the Rockies and the Sierra 
Nevada Mountains, especially in Utah and Nevada. 

From Arid to Moist. — Increased rainfall shortens the river 
cycle. The increased rainfall brings about reforestation. The 
rain and the forests restore and enlarge the rivers and make them 
in volume more nearly uniform, throughout the year. The playas 
and salinas become lakes, and salt lakes, when filled to overflowing, 
become fresh. Plants and animals gradually return and increase. 

The deposits of rock salt in regions now moist, as in New York, 
Michigan, Kansas, and Louisiana, indicate former arid conditions 
in these regions. 

From Warm to Glacial Climate. — As the climate becomes colder, 
plants and animals adapt themselves to the cooler climate, migrate, 
or perish. With increased cold and increased snowfall, more snow 
may fall than melts during the year. This occurs first upon the 
higher lands; but the areas of permanent snow gradually spread 
until the entire region is snow covered. The rivers get smaller and 



314 PHYSIOGRAPHY 

smaller and finally disappear, except near the borders of the 
glacier. 

From Glacial Climate to Warm. — The disappearance of the 
glacial ice produces floods in all rivers. The rivers become longer 
as the glaciers retreat. The melting ice leaves irregular deposits, 
whose depressions are occupied by lakes and swamps. A new 
system of drainage must be developed where the old has been 
obliterated. In general, the river cycles begin anew. Some rivers 
reoccupy their preglacial valleys in part, and in part develop new 
channels, with falls and rapids. The lower Hudson occupies its 
preglacial channel. The Genesee has developed a new channel, 
with falls and rapids at Rochester. 

QUESTIONS 

i. Make a comparative table of the characteristics of young, mature, 
and old streams. In the characteristics column, place such items as 
steepness, swiftness,. corrasion of bed with products; corrasion of banks, 
relations to falls, rapids, and lakes; profiles, divides, tributaries, trans- 
portation methods, deposition and uses to man. 

2. How, from an ordinary map, can you tell the stage of a river ? 

3. Is a muddy stream more apt to be old or young? a clear stream? 
Why? 

4. Compare the effects of elevation and of depression on the length 
of the river cycle, with examples. 

5. Do the same for a change of climate from moist to arid, from arid 
to moist. 

6. Similarly for from warm or temperate to glacial, with change from 
glacial. 

7. In what ways, and with what results, may a normal river cycle be 
interrupted? 

8. State the advantages and disadvantages to man of the different 
stages in the life history of a river. 

9. What are portages? 

10. What is it to rectify a stream? 

1 1 . Account for sunken or incised meanders. 

12. Why may a river valley be in some portions young and in other 
portions mature or old? 

13. What is imperfect drainage? 

14. What would be the effect on Lakes Erie and Ontario if eastern 
Canada should be slowly uplifted? 

15. Distinguish an estuary and a delta. 



CHAPTER XXII 
FALLS, RAPIDS, AND LAKES 

FALLS AND RAPIDS 

Location of Falls. — Falls and rapids are numerous among moun- 
tains and plateaus. They are characteristic of the upper courses 
of great rivers, and of young streams among hills. In many 
rivers falls and rapids mark the head of navigation. Small and 
light boats, like canoes, can be unloaded and carried around them; 
but large boats pass them by means of canals with locks. 

Economic Importance of Falls. — Falls and rapids furnish valu- 
able water power. This is the foundation of the manufacturing 
interests of New England. The establishment of mills at falls 
quickly develops villages, which may become flourishing cities, as 
Lowell, Rochester, and Minneapolis have done. 

Electric power developed from water power at Niagara Falls is 
transmitted as far as Syracuse, 180 miles from the falls, and this 
illustration of the use of water power at a distance from the falls 
has done much to increase the value of undeveloped falls that are 
located in mountainous regions, or where manufacturing establish- 
ments would be at a disadvantage for some other reason. 

When a fall is at the head of navigation of a river, it becomes a 
railroad center, and the loading and unloading of vessels furnish 
labor, which aids in the development of a city. 

Some Important Falls. — At Niagara Falls, the outlet of Lake 
Erie plunges over a precipice 160 feet high on its way to Lake 
Ontario. Goat Island divides the stream, making two falls; the 
larger, on the Canadian side, is called from its shape the Horse- 
shoe Fall; the smaller, the American Fall, enters the side of the 
gorge. 



3i6 



PHYSIOGRAPHY 




Fig. 158. — Niagara Falls 



FALLS, RAPIDS, AND LAKES 



317 



The enormous volume of water passing over this fall gives 
Niagara its grandeur and impressiveness and makes it one of the 
wonders of the world. 

The upper sixty feet of the face of the fall is a hard limestone, 
in nearly horizontal layers; below this is a hardened mud or shale 
with occasional thin bedded limestones, which is very easily cor- 
raded. At the foot of the Horseshoe Fall the water is some 200 







Fig. iso. — Lock ra St. Mary's Canal 

feet deep, the soft rocks at the base being worn away to this depth 
by the force with which the water strikes it and by bowlders which 
the water whirls around. Below the falls the river follows a 
gorge some seven miles long. Only a small portion of the water 
of the Niagara River is diverted from the falls for power 
purposes. 

The Genesee Falls. — The Genesee River flows < over the same 
rock formations as the Niagara, but the volume of the water is 
less, and we have here three separate falls, each of which has at 
its crest a hard limestone or sandstone and beneath this an easily 



3i8 



PHYSIOGRAPHY 



eroded shale. Below the falls the river flows through a gorge 
similar to that at Niagara, but narrower. 

St. Anthonys Falls. — The Mississippi River at Minneapolis, 
Minn., flows over a precipice capped by a somewhat thinner layer 
of limestone than that at Niagara; and as the volume of water is 
large and the cap rock was not resistant enough to preserve the 
fall, it was therefore necessary to build a wall of cement under- 



y-^u-^u A ' , ' .' , ' / ,i ,i , ' ,i ,i , i ,i |4i T v- T iy- T L 1 i- 7 i- r i-T 1 -]V\ 



■ SANDSTONE t "; -'' , - ;" ■■■■ ''■■' lFW $- Jj)ff\ 

.,"•..': SANDSTONE. 







Fig. 160. — How Niagara Falls were Formed 

neath the Fall of St. Anthony in order to preserve the falls and 
their valuable water power. Here again the river, below the falls, 
flows through a gorge several miles long. 

Shoshone Falls. — The Snake River in Idaho flows over a hori- 
zontal sheet of hard lava which overlies softer rocks, forming 
this fall. 

A Line of Falls. — In the southeastern part of the United States 
falls are found in all streams at the outer margin of the piedmont 
plateau, where they enter the coastal plain. The term fall line 
has been applied to a line connecting the falls and rapids at the 
heads of navigation in these rivers. 

How Falls are Formed. — It will be noted that in each of the 
four falls described above the cap rock of the precipice over which 
the water falls consists of nearly horizontal layers of rock that 
resists corrasion well, and that underneath this in each case is a 



FALLS, RAPIDS, AND LAKES 319 

soft rock. This is the structure which has led to the formation of 
most falls. The weak rocks are corraded more rapidly than the 
resistant rocks, increasing the slope of the stream at the point 
where the hard and soft rocks meet, until finally a fall results. 

The Meaning of the Gorge. — Many falls are, like Niagara, sit- 
uated at the up-stream end of a gorge of considerable length, 
which has been formed by the recession of the fall. It has been 
shown by careful surveys that the center of the Horseshoe Fall 
at Niagara is travelling toward Lake Erie at the rate of about 
five feet a year. Similar though less rapid recession takes place 
in all falls of this structure. 

In Fig. 160 a section of Niagara is shown. The water swirling 
around at the foot of the fall cuts backward into the face of the 
precipice, and spray, frost, and ice assist in the undermining, form- 
ing a cave. The cap rock is worn but slightly as a rule, but under- 
mining proceeds with comparative rapidity, causing the cap rock 
to fall of its own weight. The crest of the fall thus travels up- 
stream until it disappears. 

LAKES 

A lake or a pond will always be formed in all depressions in the 
land if rainfall or inflow exceeds evaporation and possible seepage. 
If the yearly rainfall and inflow exceeds the yearly evaporation 
from the surface of the lake, the lake is permanent, and water 
accumulates in the basin until it finally overflows at the lowest 
point in the rim of the basin. Temporary lakes are formed where 
the water reaching the depression temporarily exceeds the loss 
during the given time, but where the yearly supply is less than can 
be evaporated. 

Two conditions are therefore necesssary for the formation of a 
permanent lake — a basin without an outlet that reaches to the 
bottom of the basin, and an excess of water received over that 
lost. In arid regions there are many basins that are not lakes 
because of insufficient rainfall or tributary streams, but in well- 
watered regions every basin contains a lake. Lakes always indi- 
cate imperfect drainage. 



320 PHYSIOGRAPHY 

Origin of Lake Basins. — Some lake basins, Lake Superior and 
the Caspian Sea, for example, are believed to be the result of the 
uplift of intervening land masses that separated them from the 
ocean. Some of the early myths and legends of the Greeks seem 
to indicate a former passage through the Black and Caspian Seas 
to the Arctic Ocean. Other lakes are believed to be due to the 
depression of their basins. Examples of this type are Lake Baikal, 
over a mile deep, and the lakes in the Great Rift Valley, extend- 
ing from the Sea of Galilee through the Dead and Red Seas into 
the Lake region of Africa. 

Lake basins are formed by obstructing river valleys by lava 
flows, landslides, or glacial deposits. The Finger lakes of central 
New York are the unfilled portions of pre-glacial river valleys. 

Other lake basins are formed by the natural processes of rivers. 
Lake Pipin, in the Mississippi River, is formed by delta deposits 
which accumulated in its valley at the mouth of the Chippewa 
River of Wisconsin. Oxbow lakes are abandoned portions of 
streams that have been closed at one or both ends. The lakes 
along the Red River of Louisiana are made by the more rapid 
building up of the flood plain of the Red than of the flood plains 
of its tributaries. 

The craters of some dormant and some extinct volcanoes become 
partially filled with water. Such lakes are generally deep, circular, 
and with precipitous banks. Crater Lake, Oregon, is a typical 
example in the United States. Lake Avernus, on whose shores 
the ancients believed was situated the entrance to the Lower 
World, is one of several crater lakes west of Naples. Other crater 
lakes are found near Rome. In southern Germany are found older 
crater lakes, with low, gently sloping banks. 

At the base of Mt. Shasta, and several other volcanoes, lake 
basins are found in depressions between lava flows. 

Many lake basins are found in and among the uneven deposits 
of till left by glaciers. The numerous "kettle lakes," such as 
Lake Ronkonkoma on Long Island, belong to this class. In some 
instances basins have also been scoured out of the solid bed 
rock by a glacier. 




Fig. 161. — Development of Great Lakes at End of Ice Age 

Stages indicated by outlet: In Fig. i, separate; in Fig. 2, Lake Maumee into Lake 

Chicago only; in Fig. 5, through the Mohawk; in Fig. 4, through the Ottawa. 



3 22 



PHYSIOGRAPHY 




Fig. 162. — Delta Built into a Lake 
Scilvaplana, Switzerland. . 

REFERENCE TABLE OF PRINCIPAL LAKES 



NAME 

Caspian 

Superior 

Victoria Nyanza . . . 

Michigan 

Huron 

Baikal 

Erie 

Ontario 

Tchad (dry season) 
(wet season) 

Titicaca 

Nicaragua 

Great Salt Lake 

Champlain 

Dead Sea 



AREA IN ALTITUDE MAXIMUM 

SQ. MI. IN FT. DEPTH. 

.170,000 — 85 3,200 

31,200 602 1,008 

26,000 800 240 

. 22,500 581 870 

. 22,320 581 700 

. 13,000 1,700 5,600 

9,90O 573 200 

7,200 247 738 

6,000 900 8 

40,000 ... 20 

3,200 12,500 700 
2,800 

2,200 ... ... 

480 

360 — 1,268 1,300 



COMPARISONS 

California 158,000 

So. Carolina 30,000 

West Virginia 25,000 



Maryland 12,200 

New Hampshire. . . . 9,300 
New Jersey 7,800 



Delaware 2,000 

New York City 327 



Destruction of Lakes. — Lakes are temporary features in the 
early stages of the development of the drainage of a region, and 
disappear as the river system develops. Many lakes have been 
drained by corrasion of the outlet channel ; in time all lakes whose 




Plate I. Contour Map. Delta at the head of Seneca Lake, N. Y. 

From U. S. Geol. Survey. 
Scale: 1 inch = 1 mile. Contour interval, 20 feet. 



FALLS, RAPIDS, AND LAKES 



323 



bottoms are above sea level must disappear through this action, 
unless some other agent destroys them first. Other lakes have dis- 
appeared through the opening of a new outlet, for example, former 
Lakes Chicago, Agassiz, and Warren (Fig. 161) were drained when 
the melting of the glacial ice uncovered new and lower outlets. 




Fig. 163. — How Vegetation Destroys a Lake 

Pond lilies in center, smartweed at edge, farther back cat-tails, blue flags, sweet flags and 

sedges; still farther back soft turf with grass, moss, sedge and milkweed. 

A second method of destroying lakes is by filling. Some five 
miles of the southern end of Seneca Lake, New York, (Plate I) 
has been filled with sediment brought in by streams; and deltas 
are forming in nearly all lakes where streams enter, diminishing 
their size. Lake St. Clair, between Lakes Huron and Erie, has 
been greatly diminished by the growth of a delta. If sufficient 
time were allowed, this cause alone would also destroy all lakes. 

Some lakes are filled with vegetable matter; a certain kind of 
moss sometimes grows on the surface of the water and holds wind- 
blown sand and dust, which gradually spreads over the lake, 
forming a floating bog. A railroad line in Minnesota crossed such 
a bog. Cattle grazed upon it before the line was built; but the 



3 2 4 



PHYSIOGRAPHY 



engineers discovered that the floating bog was a mass four feet 
thick of moss and dust, and that beneath it was twenty feet of 
water. Eel grass and wild rice also assist in filling many lakes. 




Fig. 164. — A, Lake. B, Lilies and Bushes. C, Beginning of Sphagnous Growth 

D, Bog Climbing Hillside. E, Disintegrated Peat 

(U. S. Geological Survey.) 

Marl deposits, which form in some lakes to a depth of many feet, 
also assist in filling them. Marl consists chiefly of the shells of 
animals and the remains of lime-secreting plants. 

These methods of filling gradually convert a lake into a swamp 
or marsh, and many of our fresh water marshes are former lakes 
destroyed in this way. The student will doubtless be able to find 
examples of such marshes near his home. 



Bec-rt. 
8,000 



7,000 



6,000 



4,000 f-f 
3,000 
2,000 
1,000 



JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC." 
(P20 (0 20 10 20 10 20 10 20 IO20 10 20 10 20 10 20 10 20 10 20 10 20 


!u|l ' ... 




u 1 ' 


I -L I 1 


11 1 


tl fl B JL L j-_j_ 


119 HI 11 I 


r« ^•ni|H!r 1 m WI pi W w| 



Fig. 165. — Stream Flow Very Irregular; not Influenced by Lakes 
Discharge of Neuse River at Selnia, N. C, for 1899. 



FALLS, RAPIDS, AND LAKES 



325 



A third method of destroying lakes is by evaporation. The great 
Lake Bonneville, that once covered a part of the Great Basin as 
large as Lake Huron, was partially destroyed by evaporation. Its 
supply of rain water was cut off by a change in climate, and the lake 
shrank gradually, until to-day Great Salt Lake is all that remains. 



Sec.-ft. 
8,000 

7,000 

6,000 

5.000 

4" 000 

3,000 


JAN. 
10 20 


FEB 
10 20 


MAR. 
10 20 


APR. 
10 20 


MAY 
10 20 


JUNE 
10 20 


JULY 
10 20 


AUG. 
10 20 


SEPT. 
10 20 


OCT. 
10 20 


NOV 
10 20 


DEC. 
10 20 
































































































































































































































































































i 


1 






4 






























































■ 


J! 


L, 


k 












































■k 


mm 


1 


1 




» 


11 






















i 


s 


i 


1 


1 





Fig. 166.— Stream Flow Partially Regulated by Numerous Lakes 
Discharge of Seneca River, Baldwinsville, N. Y., for 1899. 

Summary. — When first formed, lakes will have characteristic 
shapes, depending upon the origin of their basins— round and deep 
if old craters, branched and tree-like if due to damming of rivers 
with tributaries, and with smooth parallel sides if reamed out by 
glaciers. As time goes on, streams build deltas in lakes, fill them 
with sediment, and deepen their outlets. Vegetation assists in 
filling the lake basins. Gradually the lake changes into a swamp 
that in rainy seasons is a lake, or it may become dry and form a 
playa. Again, it may become a salt lake, and ultimately change 
to a salina or salt plain. If drained, it may form a lacustrine plain. 
Such is the life history of lakes. 



326 PHYSIOGRAPHY 

Functions of Lakes. — Lakes are essentially great reservoirs of 
water. If large, they clarify the muddy waters of inflowing streams, 
so that rivers that are the outlets of lakes are generally clear. 
Even the greatest rain does not raise the level of large lakes very 
much, so that their outlets are not subject to great floods. It 
takes a long time to lower the flood level of the lake. It is for 
this same reason that in times of drouth outlets of lakes vary in 
volume less than other rivers. The Great Lakes thus regulate the 
flow in the St. Lawrence River; but the neighboring Ohio River, 
without any lake, is subject to great floods. Figure 165 shows 
the fluctuations in volume of a stream without lakes, and Fig. 
166 that of a stream of about the same average flow but having 
lakes in its course. Note the great fluctuation of the former 
and that the average summer flow of the latter is greater than 
that of the former. Large lakes ameliorate the climate of their 
vicinity, particularly on their lee side; this is because water 
changes its temperature slowly, making the lakes in summer 
cooler, and in winter warmer, than the neighboring land. 

Economic Importance of Lakes. — So important are lakes as 
reservoirs that where nature has not provided them, artificial ones 
are made. New York City, at an estimated cost of $161,000,000, 
is building in the Catskills the great Ashokan Dam for a reservoir 
which, when fed by smaller reservoirs, will eventually furnish 
500,000,000 gallons of water daily. 

The Great Lakes are of great importance as highways. The 
annual tonnage of the Great Lakes ports is about 80,000,000 
receipts and over 80,000,000 tons of shipments. In early days 
the smaller lakes were important highways. 

There is a growing appreciation of the great value of lakes as 
pleasure and health resorts. In this way they contribute much 
to the national well-being. 

QUESTIONS 

1. Why are falls and rapids incidents in the life history of a river? 

2. Discuss the advantages and disadvantages of falls. 

3. Locate the following falls: Shoshone, Missouri, Zambesi, Tequen- 
dama, Parana, Rhine, Montmorency. 



FALLS, RAPIDS, AND LAKES 327 

4. Name the cities along the fall line in southeastern United States. 

5. Explain how the steamer gets through St. Mary's Locks (Fig. 159). 

6. Discuss: "Rivers are the mortal enemies of lakes." 

7. How did fresh water Lake Bonneville change to Great Salt Lake? 

8. Explain the differences in the stream How of the Neuse, Seneca, and 
West Gallatin rivers. (See Figs. 165, 166, and 138.) 

9. Locate the following lakes: Nipigon, Ladoga, Aral, Maracaibo, 
Albemarle Sound, Malar, Baikal, Tanganyika, Balkash, Atabasca. 

10. Let each member of the class draw large scale map (1 to 1,000,000), 
based on its central meridian, of one of the principal lakes of the world 
and then make a comparative study of it based on references to encyclo- 
paedias and atlases. 



CHAPTER XXIII 
GLACIERS 

Introduction. — In almost every part of the United States snow 
sometimes falls. In the southern lowlands it may last but for a 
few hours or days; whereas in the mountains and in the northern 
section of the country it may remain for months, or even through- 
out the year. Mount Washington has snow fields far into the 
summer, and Mount Hood is perpetually snow-capped. Traveling 
northward into Canada we find more extensive snow fields at ever 
lower levels, until they finally reach sea level within the Arctic 
Circle. 

Origin. — Wherever more snow falls than disappears during the 
year, the excess accumulates; and by compression and successive 
freezings and thawings gradually changes to ice. Gra.vity causes 
the mass of snow and ice to move slowly toward sea level. These 
moving masses of ice are glaciers. 

The snow line, above which there is perpetual snow and above 
which glaciers originate, is about three and one-half miles above 
sea level at the equator, and descends toward sea level with 
increase of latitude northward and southward, reaching sea level 
within the polar circles. The more extensive the area above the 
snow line the more extensive the glaciers. 

Two types of glaciers result: those formed in valleys among 
mountains, called valley or alpine glaciers.; and those which form 
extensive ice-sheets, and known as continental glaciers. 

Distribution. — Valley glaciers are found on every continent 
except Australia, occurring in Africa and South America even 
under the equator; also upon some mountainous islands, as New 
Zealand. In North America, small glaciers occur in the United 
States in the Cascades, the Sierras, and the Rockies, increasing in 



GLACIERS 



32Q 




extent in the Canadian Rockies and in Alaska. Some of these 
glacial regions, as the Alps, Canadian Rockies, and Alaska, attract 
many tourists on account of their peculiar grandeur and beauty. 
Continental glaciers occur on all land areas where the snow line 
descends to the general level of the land. Glaciers form only on 



330 



PHYSIOGRAPHY 



land, and the ice which forms over the polar seas is not glacial ice. 
The most extensive continental glaciers are the ice-sheet which 
covers Greenland, and that which covers the Antarctic continent. 
The Greenland ice-sheet is about 500,000 square miles in area, 
while that of the Antarctic continent is greater than the United 
States in area. Several Arctic explorers have penetrated far 
toward the center of the Greenland ice-cap, and some have crossed 




Fig. 168. — Mt. Blanc, Above the Snow Line fro Right of Center) 
Snow fields ending in ice rivers that taper in size until they melt. The view is taken from across 
the valley of Chamounix. 

it. In the interior it rises to an altitude of perhaps 10,000 feet, 
with a temperature constantly below freezing, and is one of the 
most absolutely desert regions of the earth. 

Movement.— From their sources in the fields of granular snow 
and ice, called n£ve, the valley glaciers move down the valleys 
as rivers of ice, descending into the midst of forests, and even 
cultivated fields. They evaporate and melt as they move for- 
ward, becoming smaller and smaller, and finally disappear where 
the melting back just balances the forward movement of the ice. 



GLACIERS 



331 



These ice rivers behave much the same as rivers of water, erod- 
ing their beds and transporting their load of waste. Like ordinary 
rivers, too, they move faster in the middle than at the sides, faster 
at the top than at the bottom, and the line of swiftest flow lies 
nearest the convex side of a curve in the ice stream. The glacial 
river also has its rapids and falls, analogous to those in an ordinary 
river. While we know that glaciers move, the movement of most 




Fig. i6g. — A View of Glacier, to Show Streamlike Appearance, Moraines, Crevasses 



glaciers is so slow as to escape the notice of all but the most ob- 
servant. It required careful measurements to discover the manner 
of their movement. 

The Swiss glaciers move generally only a few inches a day, 
moving fastest in summer; whereas some of the Alaskan glaciers 
move as much as seven feet a day. 

Cause of Motion. — About the only thing regarding the method 
by which the glacier moves upon which all are agreed is that it 
does not move as a solid block of ice slipping down the slope. 

One explanation of its motion supposes that the glacial ice is 
granular; and that the pressure above causes the grains to move 



332 



PHYSIOGRAPHY 



on and over one another. This may be illustrated by the movement 
of moist brown sugar when piled up. 

Another explanation attributes the movement to alternate 
freezings and thawings. The great pressure above causes the ice 
to melt, particle by particle. Each particle, as water, occupies 
less space, the pressure upon the particle is decreased, and the 
particle freezes again at a lower level, only to be re-melted by the 
pressure. The glacier movement is thus the sum of the move- 
ments of its grains. 

A simple experiment may be made that suggests the proba- 
bility of each of the above explanations: (i) Take a long block of 
ice, and rest the ends upon supports. After some time the block 
will be bent, much as a thin board supported in the same way, 
although the surrounding temperature is below freezing. (2) Sup- 
port a block of ice by a fine wire around it. The wire will quickly 
cut through the block, the two pieces again freezing together below 
the wire, even though the temperature is above freezing. The 
pressure of the block melts the ice, and the water thus formed im- 
mediately freezes again with release of pressure. 



a 5 s- c 



B T T D 

Fig. 170. — Representation of Movement of a Glacier 

The strip 5 T changes to S' T'. Because ice cannot stretch, ittends to crack at right angles to 

the line from T' to i', so that the crevasses formed point obliquely up stream. 



Effect of Movement. — As a result of glacier movement, the snow 
is slowly drained away from the mountain slopes. Because of the 
unequal movements in the glacier it becomes very much broken. 
These breaks are called crevasses. 

Where a glacier passes from any slope to a steeper, the bend- 
ing and the more rapid movement causes a series of transverse 



GLACIERS 



333 




Fig. 171. — Nisqually Glacier, Mount Rainier, Washington 
Five miles long and in places nearly one mile wide. See also Fig. 225. 

crevasses to form across the glacier. They are more common in 
the upper courses of glaciers, although the Rhone glacier ends in 
such an ice rapid. If the slope becomes again less steep, these 
crevasses disappear by closing up and melting of the surface. 

Because of the more rapid movement of the glacier at the mid- 
dle than at the sides, there develop a series of oblique cracks, 



334 



PHYSIOGRAPHY 




Fig. 172. — Crevasses in the Eiger Glacier 
Note the surface moraine and the rope around the men. 



which become ever wider as they advance down the slope. They 
are the lateral crevasses, and they make walking upon the lower 
courses of glaciers very difficult and dangerous, especially when 



GLACIERS 



335 



the winter snows have temporarily bridged them over. Some- 
times a glacier passes from a narrow to a wider valley, and the 
ice, spreading out laterally, produces longitudinal crevasses. 

Glacial Mills. — The surfaces of most valley glaciers are too 
much broken to permit the formation of streams upon them from 
the melting ice in summer; but occasionally such streams are 




Fig. i73- 



-Glacial Scratches (toward us), Glacial Bowlders, and Pothole 
Glacier Garden, Lucern. 



formed. These sooner or later tumble into a crevasse, and armed 
with the bowlders and finer materials which also find their way 
there, grind depressions in the bed rock beneath the ice. These 
are glacial mills, and their grist is the materials which serve them 
as tools. Larger and more numerous streams form on continental 
glaciers, and the pot holes ground out beneath them are larger. 
Work of Glaciers. — Glaciers drain away precipitation in the form 
of snow, and like rivers, corrode their beds, transport their load of 
waste, and when they melt deposit it. 

(a) Drainage. — An area about equal to that of the United 
States is drained by continental and valley glaciers. 



336 PHYSIOGRAPHY 

(b) Corrasion. — Ice, like water, has little power to corrade; but 
when supplied with rock waste imbedded in its under surface, it 
becomes a powerful agent of erosion. The weak and weathered 
portions of the bed rock are removed, and the fresh and harder 
portions rounded, striated and grooved, the strice and grooves 




Fig. 174. — Upper Grindelwald Glacier 
Glacial scratches proceeding from under the ice on the left side. 

being parallel to the direction of movement of the ice. Such 
rounded masses of the bed rock are called roches moutonnes. 

Valley glaciers ream out their valleys, changing V-shaped val- 
leys to the U-shape. Continental glaciers plane down the up- 
lands, rounding off the irregularities of the hills and ridges that 
may end in cirgues, great amphitheatres surrounded by high cliffs. 

(c) Transportation. — All glaciers carry waste. The continental 
glacier gets its load from the surface over which it moves; the 
valley glacier gets the greater part of its load from the bordering 
slopes. Whether carried upon the surface of the glacier, within 
the ice or beneath it, the materials are known as moraine. Mate- 



GLACIERS 



337 



rials along the sides of a valley glacier constitute the lateral 
moraine; that beneath the ice the ground moraine; and that 
about the end of the glacier the terminal moraine. When two 
glaciers join, the united lateral moraines between them, con- 
tinued upon the glacier below the junction, is the medial moraine. 
If the medial moraine is abundant, it may so protect the ice be- 




Fig. 175. — Famous Rosegg Glacier 
Showing tongue of ice with crevasses, moraines, and ice-born stream. 



neath from melting that the morainic ridge may stand up fifteen 
or twenty feet above the general surface of the glacier. Large 
slabs of stone are often left perched on pedestals of ice by the 
melting of the ice around them. Such perched stones are known 
as glacial tables. 

(d) Marking. — While the materials in the ground moraine are 
subjected to the crushing weight of the glacier, the bowlders and 
pebbles in it being often polished and straited, the materials of the 



338 



PHYSIOGRAPHY 




Fig. 176. — Cross Section of a Glacier 
Showing lateral and ground moraine, crevasses, and ice table. (Walther.) 

moraines are, for the greater part, not rounded as are those car- 
ried by rivers. 

(e) Deposition. — When a glacier melts, its load of waste is de- 
posited — not in layers and assorted, as is the waste carried by 
rivers, but pell-mell, without trace of assorting. (Figs. 178 and 
181.) 

The terminal moraine marks the limit reached by the glacier. 
Since glaciers vary their rate of movement with the season, a 




Fig. 177. — Two Views of Same Glaciated Pebble of Limestone, from Chicago 
The 18 facets indicate alternate fixity and change of stone in the ice. 



GLACIERS 



339 



series of concentric ridges may mark successive retreats of the 
glacier front. These may later be overrun by the glacier during 
a period of extension down the valley. The retreat of the ice- 
front is in no sense a backward movement of the ice, as ice- 
movement is always down the valley or slope; it means only 
that the rate of forward movement does not equal the rate of melt- 
ing back, and the ice-front takes a position farther up the valley. 




The Sub-glacial Stream. — From the end of every valley glacier, 
and at frequent intervals along the front of the continental glacier, 
issues a sub-glacial stream. These streams, supplied chiefly from 
the melting ice, are much stronger in summer than in winter. 
They are usually muddy from the load of rock flour they bear, 
derived from the ground moraine. 

In valley glaciers the sub-glacial streams usually deposit their 
load in some lake or river; but in continental glaciers they often 
build extensive apron-like deposits in front of the glacier. These 
deposits along the edge of the ice-sheet are known as outwash 
plains. Where glaciers reach the sea, the sub-glacial stream may 
issue beneath the sea level; and the glacier, instead of melting 
back, may break off in blocks and float away as icebergs. 

Sometimes the channels of streams beneath ice-sheets become 
clogged with pebbles and coarse sand. Such a ridge, exposed by 
melting back of the ice-front, is known as an esker. The deposits 



340 PHYSIOGRAPHY 

sometimes made at the edge of the ice-sheet, in part the work of 
the sub-glacial stream and in part from the melting ice, and 
showing a sort of stratification, are known as kames. 

Former Extension of Glaciers. — As we may trace the shore line 
of a lake that has disappeared, by the characteristic shore line 
features, so we may recognize the former existence of glaciers in 
regions where now no glaciers are found. Glacial records are so 
characteristic as to be usually unmistakable. We observe the 
work of the glaciers now existing, and we know that glaciers of the 
past did the same sort of work. Therefore, when we find polished 
and striated surfaces on the valley sides far above the present 
glaciers in the Alps, we do not hesitate to expand our glaciers to 
those heights; and when we find U-shaped valleys in any region, 
though now far removed from any modern glacier, in imagination 
we restore the glacier, for ice alone seems competent to make 
U-shaped valleys. 

Thus extended, the Alps become a very much larger glacial 
region, extending to the plains of northern Italy; and the minia- 
ture glaciers now found in the United States become the centers of 
similar regions. 

Of even more interest, and of much greater economic importance, 
is the former extension or existence of continental glaciers. While 
doing the same sort of work as valley glaciers, the records made by 
continental glaciers are more varied and more enduring. These 
records are continent wide, and may be read alike in the planing 
down of the highlands, and in the filling and leveling up of the 
lowlands. Reading the records we discover that there was a time, 
in the not distant past as earth-time is measured, when much of 
northern Europe, extending to and including the British Isles, and 
most of North America down to the latitude of New York City, 
were covered by continental ice-sheets. This time is known as 
the Ice Age or Glacial Period. Many other parts of the earth have 
had glacial climates, some of them probably several times. 

The Ice Age in North America. — If we travel across the United 
States from north to south we are impressed by the unlikeness of 
the topography in the north and in the south. 



GLACIERS 



341 




Fig. i7g. — The End of the Grindelwald Glacier 

Occupying the bottom of a great U-shaped valley. Note the cirque-like cliffs at the base of the 
Viescherhorn in the background. The cliff on the right, smoothed to the top, indicates a 
much more extensive glacier here in former times. 



In the north the rivers are young, often having rapids and falls; 
and lakes are numerous. The uplands are level; or, if uneven, the 
hills and ridges are covered with a cloak of unassorted and usually 



342 PHYSIOGRAPHY 

coarse mantle rock. Numerous bowlders, wholly different from 
the bed rock of the region, are widely scattered, especially in the 
east. The mountains have numerous lakes and swamps in their 
valleys. The soils are all transported, being entirely unlike the 
decomposition products of the bed rock. 

In the south the rivers of the upland are mature, having long 
ago removed their rapids and falls. There are no lakes or swamps 




Fig. 180. — Typical Rounding by Glacial Action 
Note erratics deposited by the glacier. U-shaped valley in background. Kerguelen Island. 

(Penck.) 

in the mountains or in the uplands; and the uplands are hilly and 
cloaked with residual soils. 

The line which separates these two types of topography follows 
roughly the Missouri and Ohio rivers to their sources in the 
Rocky Mountains and in southwestern New York; thence west- 
ward by an irregular line to Puget Sound and eastward and south- 
eastward through New York City to the east end of Long Island. 
This line marks the southern limit of the ice during the Glacial 
Period, and is the southern boundary of the deposits made by the 
continental ice-sheet. 

As the ice-sheet moved down from the north, it invaded a region 
probably about as maturely dissected as Kentucky and Tennessee 



GLACIERS 



343 



now are. River systems were widely branching, and lakes had 
disappeared. When with change of climate the ice-sheet began 
to melt back, a land surface wholly changed was revealed. Ridges 
were planed down, and valleys partially or wholly filled. Wher- 
ever the ice-sheet paused for a time in its retreat, there was formed 




-Typical Unassorted Drift near Lake Grinnel 
(N. J. Geological Survey.) 



a terminal moraine. If the ice advanced for a season, former 
moraines were obliterated, to be succeeded by new when again 
retreat began. The ice-sheet did not advance and recede equally 
along its entire front, and records of various advances and retreats 
remain. Many terminal moraines or halting-places, roughly 
parallel, are found between the Ohio River and the Great 
Lakes. 

In melting, the ice often left great bowlders perched in unstable 
positions. Such bowlders are often rocking stones, and are unques- 
tioned work of the ice, as running water would not leave them thus. 



344 



PHYSIOGRAPHY 




By overruning the 
ground moraine, long, 
lenticular hills, called 
drumlins,were fashioned. 

The general name for 
all deposits left by the 
ice is drift; and while 
valleys parallel to the 
direction of ice move- 
ment were often kept 
free of drift, or even 
deepened by the ice, val- 
leys transverse to the 
direction of movement 
were generally drift- 
filled. 

From the time of the 
advance of the ice-sheet 
south from the present 
position of the Great 
Lakes, then probably a 
river valley, until its re- 
treat north of them, all 
drainage was southward 
to the Gulf or eastward 
to the Atlantic. The 
Great Lakes themselves 
developed outlets south- 
ward to the Mississippi 
when first freed from 
the ice. 

With retreat of the ice- 
front northward from 
the divide between the 
Hudson Bay and Gulf of 
Mexico drainage basins, 



GLACIERS 



345 



a great lake formed, which developed an outlet now occupied 
by the Red River of the North. With the melting of the ice dam 
this lake disappeared, and its silt covered bed is now one of the 
greatest wheat producing regions in North America. To this 
ancient glacial lake the name Agassiz has been given. 




Fig. 183. — Pothole, Bronx Park, New York City 

Made in glacial times; the stone in it beside the tree is an erratic from the Palisades. 

(Martin Steljes, Photographer.) 



Further retreat of the ice-sheet revealed the more favorable 
route eastward through the Mohawk Valley, and the southward 
outflow of the Great Lakes ceased. The Mohawk route was in 
turn abandoned for the present route of the St. Lawrence, when 
the glacier had sufficiently melted. 

With the melting of the glacial sheet all rivers issuing from its 
front were swollen beyond their usual volume, and beyond the 
capacity of their ordinary channels. The burden of rock flour 
carried by these rivers was thus spread along their banks as nat- 



346 PHYSIOGRAPHY 

ural levees, forming a peculiarly fine and even-textured deposit 
known as loess. Great thicknesses of loess are found along the 
Missouri River at and below Kansas City, and along the Mississippi 
southward as far as Baton Rouge. That it was deposited at least 
in part by the rivers is indicated by the occurrence in it at Vicks- 
burg and elsewhere of numerous snail shells; also by its gradual 
thinning and final disappearance in a few miles back from the 
river front. Similar deposits in Germany and China have been 
attributed in part to the wind; but the glacial origin of the mate- 
rial is undoubted. 

Lakes and Marshes. — Mention has already been made of the 
occurrence of numerous lakes in the glaciated area of the United 
States, and of their absence south of this area; and the origin of 
some of the lake basins has been suggested. Lake Agassiz was 
perhaps the least common type of glacial lakes, although the lakes 
produced by temporary ice dams were perhaps the most exten- 
sive. The Great Lakes, which individually were probably in 
basins in part produced by glacial drift interrupting the drainage 
in a pre-glacial valley, were for a time united into one greater 
lake by the ice-sheet blocking the outlet through the St. Lawrence. 
While all were united, and stood at a common level, they were 
separated into three somewhat distinct basins by the high land 
south of Georgian Bay, which rose as an island in the midst of 
their icy waters. To the Superior-Huron-Michigan division the 
name Algonkian has been given; greater Ontario has been called 
Iroquois; and modern Erie bears the name of its larger ancestor. 

The margins of these greater lakes have been traced by the 
beaches and other shoreline features then developed. With the 
melting of the ice these lakes gradually assumed their modern 
proportions. For their development see Fig. 161. 

Other larger lakes, like the Finger Lakes of New York, and most 
of the lakes of northern New York and New England, occupy 
basins produced by drift deposited in valleys, and represent the 
unfilled remnants of pre-glacial rivers. 

Westward from New York, and north of the Great Lakes, by 
far the most numerous type of glacial lakes occupy local depres- 



GLACIERS 347 

sions in the drift. As the ice-sheet receded it left an uneven sur- 
face, and in the depressions lakes were formed. These were often 
shallow, and quickly changed to the marsh stage; and in this way 
the numerous high-level marshes of the northern United States 
and Canada were formed. 

Still another type of lake was produced near the southern limit 
of the ice-sheet when the glacier came down to the sea. In south- 
eastern New York, and along the New England coast, deep and 
sometimes circular lakes occur. These kettle lakes probably 
represent the resting place of a detached mass of ice over which 
drift was deposited. Shallower lakes of the same type are rush- 
filled, or have become dry lake beds. 

Economic Importance of the Drift. — The presence of the drift in 
the northern section of our country has played an important part 
in determining the lines of its economic development. The general 
result of the deposit of the drift was to leave this region more nearly 
level than before the coming of the glacier. This has favored the 
building of roads and railroads in the section, which in turn pro- 
moted commerce. 

A deeper covering of mantle rock is found in the glaciated than 
in the unglaciated regions, and this favors the more even and con- 
stant flow of rivers. The numerous lakes here also equalize the 
flow of streams, and transportation by water is made possible. 
River transportation in the South is both local and limited; whereas 
in the North our lakes, our rivers and our canals make carriage by 
water second in importance to carriage by rail. 

Mining in the drift-covered section is of little importance, since 
the thick coat of drift has made the discovery of important min- 
eral deposits difficult. With the exception of oil, gas, and salt, 
important mineral deposits have been discovered and developed 
only where the drift covering is thin or where streams have cut 
deep valleys. 

The soils of the two sections are very unlike, but it is difficult 
to determine whether the drift has furnished a better or poorer 
soil than would have developed from the bed rock beneath. In 
the eastern part of the drift-covered section the soils are too coarse 



54 8 



PHYSIOGRAPHY 






""& 







■ J , 





< rt 



and sandy, and the surface is cumbered with glacial bowlders; 
but farther West the soils are fine textured, free from bowlders, 
and very productive. It is probable, however, that the difference 
in character of crop raised in the two sections is more a difference 
due to climate than to difference of soil. 
The economic products obtained from the drift are clay, sand, 



GLACIERS 349 

and gravel. The clays are manufactured into bricks, tiles, and 
crockery; the sands into glass and brick ; and the gravels are used 
for road-making. 

Causes of the Glacial Period. — Much speculation has been in- 
dulged in regarding the causes of glacial periods. No single cause 
has been generally considered competent. Glacial conditions now 
exist on high mountains in all latitudes; in high latitudes even down 
to sea level. 

These facts have suggested the two hypotheses most acceptable. 
The one supposes that glacial climates were produced by the eleva- 
tion of extensive land areas. The elevation of the regions east and 
west of Hudson Bay, the centers of accumulation of the ice during 
the last glacial period, by only a few thousand feet would make 
those regions again glacial centers. A glacial sheet once formed, 
the temperatures about its borders are made lower, and the sheet 
extends. It would thus require an elevation no greater than has 
frequently occurred to bring another glacial sheet to northern 
United States. 

The other hypothesis makes the development of glacial sheets 
the result of increased length of winter combined with increased 
distance from the sun. At present our winter occurs when we are 
3,000,000 miles nearer the sun than we are in summer, and more- 
over our winter is about seven days shorter than our summer. 
The shape of the orbit of the earth changes, becoming less nearly 
a circle, and the position of the earth's axis so changes that there 
comes a time when we are farthest from the sun in winter, and our 
winter is longer than our summer. This hypothesis makes our gla- 
cial climates the result of long aphelion winters. According to this 
hypothesis, glacial conditions now exist in the southern hemisphere; 
and the fact that the only extensive land area in southern latitudes 
is covered by a thick sheet of ice seems to support the hypothesis. 

Still a third hypothesis is that the changes in climate that pro- 
duce glacial ice-sheets, and later melt these same sheets, are largely 
the result of a change in the amount of carbon dioxide and water 
vapor in the air. With decrease in the percentage of these constit- 
uents the climate grows colder, and with increase, warmer. This 



350 PHYSIOGRAPHY 

hypothesis would bring glacial climates to both hemispheres at the 
same time, instead of alternately, as required by the second hypoth- 
esis stated. 

QUESTIONS 

i. What fractional part of the land area of the globe is now drained 
by glaciers? How does this area compare with that of the United 
States? 

2. What area has been, but is no longer, so drained? 

3. Name and locate fifteen mountainous regions with glaciers. 

4. Discuss the effects of three elements entering into the explanation 
of movements of glaciers. 

5. Apply to the explanation of the directions of different kinds of 
crevasses the statement that "ice cracks at right angles to the line of 
strain." 

6. Tabulate a careful comparison of glaciers with streams, as to move- 
ment, works, deposits, etc. 

7. "Crag and tail" — Which side (or sides) of a glaciated hill has bare 
crags and which side has a tail of stones? Why? 

8. Discuss the effects of glaciers or ice-sheets in passing over valleys 
at right angles to the direction of ice movement. On valleys parallel 
to ice movement. 

9. In what direction will the top of an ice table finally tip, and why? 

10. Why are there almost no lakes in Pennsylvania and so many in 
New York? 

n. Why is the land in southeastern Ohio so hilly and in northwestern 
Ohio so nearly level? 

12. Most New England fences are built of stone. Why? 

13. Explain how soil left by a glacier may be better than the soil 
removed from the same place by the glacier. How glacial soil may 
be worse than the soil removed. 

14. Which explanation of the causes of the ice age seems to you the 
most probable? Why? 

15. Will northern United States be again covered with an immense 
ice-sheet? 



CHAPTER XXIV 
PLAINS AND PLATEAUS 

The Earth a Spheroid of Rotation. — The form which gravitation 
gives a liquid or a gaseous mass is that of a sphere. If the mass 
is in rotation while in this condition, it becomes a spheroid, flattened 
more or less at the extremities of its axis, as the rate of rotation 
is greater or less. The earth is such a spheroid, flattened at 
the poles just the amount required by the present rate of rotation. 

The fact that the earth is a spheroid of rotation does not prove 
that it assumed its form while in a liquid or gaseous state. The 
water of the ocean would assume a spheroidal form because of 
rotation, whatever the shape of the solid parts, and since the 
ocean is the base level of erosion, all land would in time conform 
to its shape. Thus, in a general way, the ocean determines the 
flattening at the poles and the form of the earth. 

Relief. — The relief features of the earth are departures from the 
perfect spheroid of rotation. The highest points of the .continental 
masses rise a trifle less than six miles above sea level, and the 
deepest parts of the ocean are about six miles below. The extreme 
departure from a perfect spheroid is therefore twelve miles, which 
is less than one three-hundredth of the radius of the earth. This 
slight irregularity gives us land on which to live. 

The relief features of the land are much smaller departures 
from the perfect spheroid than those forming the ocean basins 
and continents, yet they modify the climate of the region, its 
adaptability to agriculture, and the habits and occupations of its 
inhabitants. 

The large relief features of the land are plains, plateaus, and 
mountains. 



352 PHYSIOGRAPHY 

Plains. — A plain is a broad, relatively smooth surface that nowhere 
appears conspicuously higher than adjoining land or water. 

The bed rock below plains usually has horizontal strata, but this 
is not necessarily the case where plains have been formed by a 
glacier, like that of northwestern Ohio, neither is it necessarily 
the case in deltas, flood plains, lava plains or plains of denudation. 

The Formation of Plains. — The smooth surface of most of the 
true plains is due to the fact that the material forming the 
surface was deposited as sediment in water. Such deposits 
are always smooth, and have a nearly horizontal surface. The 
only important plains formed by other processes are those formed 
by glaciers, where they have smoothed the rock and covered it 
with a level deposit of till, and old lands worn down almost to 
sea level. 

The plains formed by deposition possess characteristics which 
depend upon the body of water in which the deposition occurred, 
and the sea, the lake, and the river each contributes its own type. 
Such plains are known respectively as marine, lacustrine, and allu- 
vial plains. 

River deposits become flood plains upon the subsidence of the 
river, and lacustrine deposits become plains through the de- 
struction of the lakes. Marine deposits may become marine 
plains either through the uplift of the land or the subsidence of 
the sea. There are many evidences that changes of relative level 
of land and sea are now in progress; for example, a Spanish powder 
magazine, built near New Orleans during the eighteenth century, 
is now under water; certain orchards along the New Jersey coast 
are now submerged; and the Temple of Jupiter Serapis, near 
Naples, Italy, which is known to have been on dry land in 235 
a.d., and which was rediscovered in 1749, was found to have been 
submerged between those dates to a depth of 21 feet and to have 
been elevated again. The double caves in Fig. 185 show that the 
California coast has been uplifted. Both caves were cut by the 
waves. These illustrations indicate recent changes of level. The 
finding of the skeleton of a whale in the glacial gravels near Lake 
Champlain, indicates an earlier change, and the remains of sharks' 



PLAINS AND PLATEAUS 



353 



teeth and other marine animals in the sedimentary rocks of our 
mountains, indicate still earlier changes. Finally, it must be 
recognized that the very existence of dry land is evidence of change 
of relative level of land and sea, for without it erosion would long 
since have reduced the land to sea level. Each of these changes 
was necessarily accompanied by a change in the location of the 
shore line. 




Fig. 185. — Two Caves Near Mallagh Landing, Cal. 

The upper cave was formed when the land was ten feet lower than now. The lower one is 

now being formed. 



Some of the changes may have been due to depressions of the 
ocean bottom, which would allow the water to settle away from 
the land; others to the accumulation of sediment, or lava, on the 
sea bottom, which would cause the water to overflow the land; 
still others to the withdrawal of the sea water to form a continen- 
tal glacier; and yet others to the uplift of the lands. 

Such changes have repeatedly exposed fresh areas of the ocean 



354 PHYSIOGRAPHY 

floor, forming marine plains, or have tilted land, draining lakes 
and forming lacustrine plains. 

Marine Plains. — Marine plains are those composed of sediments 
deposited in the sea. Near the shore the sea bottom is almost 
everywhere white sand which has been freed from the softer par- 
ticles of rock waste under the vigorous action of the waves, and 
consists almost entirely of quartz. Beyond the sand deposits of 
mud are formed, which are composed of exceedingly fine particles 
of the softer and more perfectly decomposed minerals. Deposits 
of this kind border all continents, and form continental shelves. 
When a portion of the continental shelf becomes dry land it is a 
marine plain, and when near a coast it is called a coastal plain. 

Plains thus formed consist of gently sloping strata of gravel, 
sand, and mud, varying as the conditions under which they were 
deposited varied. 

Some of the strata of the marine plain are porous and allow 
ground water to flow through them readily; others are impervi- 
ous. These conditions make possible the numerous artesian wells 
bored in the marine plains bordering eastern United States. 

The Atlantic Coastal Plain. — Bordering the Atlantic Ocean from 
New York to Florida and the Gulf from Florida to Mexico is a fine 
example of a coastal plain, that in some localities is a hundred miles 
wide. See Fig. 186. 

That it was formed under the sea and was a part of the con- 
tinental shelf is shown by the remains of marine animals and plants 
found in the strata, and by the fact that the strata are of the 
same kind as those now forming the ocean floor and are often 
continuous with them. That the region has only recently been 
raised above sea level is also shown by the numerous marshes, 
the unconsolidated deposits, and by the simple drainage. The 
streams of the outer portion have few tributaries, their valleys 
are but slight depressions, and the regions between them are so 
level and sandy that most of the water sinks into the ground. 
Fig. 1 86 shows the flatness of this portion. On the inner half of 
this plain the valleys are deeper, the streams have more tributaries, 



PLAINS AND PLATEAUS 



355 



and the drainage is more mature. The marshes have been drained 
and well-defined divides begin to appear. These conditions are 
the natural results of the slow elevation of a marine plain, and con- 
trol the industries of the regions. 

In the Carolinas rice is raised in the marshes. Between the 
marshes are wide areas of sand, of little value for agriculture, 



•• 




i r m lift 


■ 1 


■ 





Fig. i86. — The Great Pine Plains of Southern New Jersey 
A part of the Atlantic Coastal Plain, much of which is like this. 

which are chiefly occupied by pine forests. Farther inland the 
soil is fertile, and much cotton is raised; and in certain localities 
gardeners maintain successful truck farms. 

At the western border of the Atlantic Coastal Plain the land 
rises somewhat abruptly to the Piedmont Plateau. The rivers 
of this region usually have falls or rapids where they descend from 
the plateau to the plain, which furnish water power and mark the 
head of navigation. 

Because of these conditions many important cities have grown 
up along the inner margin of the plain. A line connecting these 
cities, called the " fall line," marks the approximate location of 
the shore line while the strata forming the coastal plain were being 
deposited. Among the important cities located along this line 



356 PHYSIOGRAPHY 

are Trenton, N. J., Philadelphia, Pa., Washington, D. C", Rich- 
mond, Va., Raleigh, N. C, Camden, S. C, Columbia, S. C, and 
Augusta, Ga. 

Ancient Coastal Plains. — Most of the sedimentary rock of the 
world is of marine origin, and it is probable that when every 
region of the world where the bed rock was formed from marine 
sediments first appeared above the sea, it was a coastal plain. 

It is evident, however, that only those that recently became 
dry land can be properly called coastal plains, because erosion 
would have long since dissected the original surface of the older 
ones unless some of the other methods of plain making had been 
in action, and in this case the plain should not be classified with 
coastal plains. A region in Wisconsin, and another in western 
New York, were doubtless at one time coastal plains. 

Lacustrine Plains. — When a lake is destroyed either by drain- 
ing, filling, or by evaporation, the former lake bottom becomes a 
lacustrine plain. Such plains are always small compared with 
marine plains, their strata are nearly horizontal, the surface is 
level, and the soil, as a rule, is more uniformly fertile and of finer 
texture than that of the coastal plain. 

If the lake was quickly drained, as by the melting of a portion 
of a glacier which dammed an outlet, the margin of the former 
lake would be marked by deposits of sand and gravel of greater 
value for building purposes than for agriculture. 

The inner portion would consist of muds brought in by streams, 
and would be very fertile. If a lake was slowly drained or filled, 
and supported a large growth of eel grass or marsh grasses, the 
soil of the resulting plain would be likely to be of uniform fertility 
throughout its whole area. Where lakes have been destroyed by 
evaporation a level surface results; and since only water evapo- 
rates, all of the dissolved mineral matter that was in the water 
is deposited on the plain. Such plains are called salinas. They 
are not fertile, but often contain valuable deposits of salt, soda, 
and borax. 

In Bolivia there is a salina several thousand square miles in 



PLAINS AND PLATEAUS 



357 



area — a level, white plain, covered by a layer of salt four feet 
thick. In the Great Basin (U. S.) are many deposits of minerals 
formed in this way. 

The Valley of the Red River of the North. — One of the most 
level regions of the world and one of the greatest lacustrine plains 
is the Valley of the Red River, which flows north between Minne- 
sota and North Dakota. It is the floor of former Lake Agassiz, 



**,"-** **-_M HkjUJfgjt- 




■■■■ 



H Hi 1 1 



Fig. 187. — The Lacustrine Plain 
In the Valley of the Red River of the North. The points on the sky line are houses. 



which existed while the continental glacier blocked the drainage 
lines toward the north. The soil here is fine and rich, and produces 
enormous quantities of excellent wheat. 

Lake Bonneville. — Great Salt Lake is a shrunken remnant of a 
greater lake known as Lake Bonneville, which once occupied the 
eastern portion of the Great Basin. The sediments deposited in 
this lake, which was as large as Lake Huron, filled the valleys 



358 



PHYSIOGRAPHY 



between north and south mountain ranges, forming many small 
lacustrine plains. Fig. 188. 

Other Lake Plains. — In several places along the south shore of 
the Great Lakes large bodies of water accumulated, toward the 
close of the Glacial period, between the ice front and the high 
land to the south. The sediments deposited in these lakes filled 
the irregularities in the lake bottom, and when the water disap- 
peared with the melting of the ice, a number of important lacus- 




Fig. 188. — The Floor of Ancient Lake Bonneville in Utah 
The hills and mountains are nearly buried by accumulated sediment. 

trine plains were exposed. The prairies of northern Illinois were 
once covered by the waters of Lake Chicago, an extension of Lake 
Michigan, and are now lake plains. In New York State, a 
southern extension of Lake Ontario gave us the lacustrine plain 
that extends from the Mohawk Valley to Syracuse. This plain 
provided a favorable location for the Erie Canal and the New 
York Central Railroad. It was also used by the early settlers of 
the West as the main highway toward their new homes. 

Some of the former lakes of the group of Finger Lakes of 
New York State are now lacustrine plains; one of them, the 
valley of Mud Creek, just west of the Canandaigua Lake, is six- 
teen miles long and from one-half to one mile wide. It is level 
and is one of the most fertile regions of the State. 



PLAINS AND PLATEAUS 



359 



River Plains. — The leveling action of water is nowhere better 
shown than in a river valley. All the deposits of a river have the 
nearly horizontal surface of the plain, because of the leveling 
action of the floods which sometimes cover them. The principal 
classes of river plains are flood plains and compound alluvial fans. 

The materials forming the flood plains are not arranged in con- 
tinuous horizontal strata, but are exceedingly irregular, owing to 



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Fig. 189. — The Flood Plain of the Grand River at the Mouth of the Gunnison, 

Grand Junction, Col. 

The Grand River flows through a barron region, much of which is reclaimed by water 

led from the Grand by irrigating canals. 

the meandering of the channel and to the fact that during floods 
a river often corrades new channels to great depths. As the flood 
subsides, the depressions thus formed are rilled with layers of 
sediments, which sometimes differ from those eroded in fineness 
and also in inclination. In this way the nearly horizontal original 
deposits of flood plains are cut away first in one place and then in 
another. 

The surface of the flood plain is usually higher near the channel 
than elsewhere, through the building of natural levees. As the 
stream shifts its course, old levees and abandoned channels inter- 
rupt its level surface; but these slight irregularities do not affect 



360 PHYSIOGRAPHY 

the level appearance of the plain, as is clearly shown in Fig. 189 
of the Flood Plain at Grand Junction, Colorado. 

Economic Importance of Flood Plains. — The soil of flood plains 
is rich in plant food and is easily tilled. The large proportion of 
silt in the deposits makes the capillary distribution of the ground 
water well nigh perfect, and since the water table is usually near 
the surface, flood plains rarely suffer from drought. The neigh- 




Fig. 190. — The Flood Plain of the Canadian River, Oklahoma 

boring river provides an easily traveled highway, which makes 
flood plains exceptionally accessible. These two characteristics, 
fertile soil and accessibility, have made flood plains so desirable 
for settlement that they are nearly everywhere densely populated. 

The flood plain of the Mississippi below the mouth of the Ohio 
is from 20 to 50 miles wide and about 600 miles long. Memphis, 
Vicksburg, and Baton Rouge are located on the plain, and fine 
crops of corn, cotton, and sugar-cane are raised there. 

The flood plain of the lower Rhine is one of the most densely 
populated and carefully cultivated regions of Europe. The flood 
plain of the Yellow River, in China, probably has a denser popula- 
tion than any other region in the world. 



PLAINS AND PLATEAUS 



361 



The advantage which the less strenuous struggle for existence 
gave the ancient inhabitants of flood plains over the inhabitants 
of less favored regions, is shown in history. Egypt developed on 
the flood plain of the Nile, and Chaldea and Babylon on the 




Fig. 191. — The Ohio River at Flood Stage 
New Albany, Ind. The streets of the town are under water, and boats replace carriages. 

plains of the Euphrates and the Tigris. These nations were so 
important among the ancients that the period prior to 800 B.C. 
is sometimes mentioned as the " fluvial period " of history. 

The chief objection to life on flood plains arises from the danger 
of floods. Fig. 191. 

In 1897 the Mississippi flooded 13,000 square miles of its lower 
flood plain, destroying property valued at $15,000,000. In 1903 
a flood in the Ohio destroyed $40,000,000 worth of property. 

Among the most disastrous floods are those of the Yellow 
River of China. In 1897, 50,000 square miles of its flood plain 
were inundated, covering many villages. More than 1,000,000 
people were drowned, and an equally great loss of life from 



362 PHYSIOGRAPHY 

famine and disease followed the flood. During one of its floods, 
that of 1902, the Yellow River shifted its course so that it emptied 
into the Gulf of Pechili, 300 miles north of its former mouth in 
the Yellow Sea. 

The disastrous effects of floods may be prevented by building 
artificial levees or dikes along the banks. This has been done 
along a large part of the lower Mississippi, and the lower Rhine 
has not only been confined within high banks, but its course has 
been " corrected," or straightened. 

Very extensive dikes have been built along the Po River by the 
Italian Government. A line of master dikes, intended to confine 
the river during the highest floods, is built on each side of the river 
for long distances, and between them in many places are secondary 
dikes which confine the river during all except the highest stages 
of water. More than 1,000 miles of such dikes have been built 
along the Po and its tributaries. 

This treatment will prevent floods if the levees are sufficiently 
high and strong; but it also prevents the annual contribution to 
the fertility of the soil which, the floods bring, and there are regions 
where the inhabitants prefer to let the floods spread over the flood 
plains. In such localities buildings are located on the higher 
lands. Loss of life will be prevented in a large measure if the 
inhabitants are warned of the danger of the flood. The United 
States Weather Bureau is devoting special attention to this sub- 
ject, and is able to give people warning of the approach of a flood 
and to tell them the probable stage of the water. 

Peneplains. — When erosion has been long continued in a region, 
all elevations are gradually worn down toward base level. One 
after another they disappear, and if sufficient time were allowed, 
doubtless even the hardest rocks would reach base level, produc- 
ing a featureless plain. 

It is not probable that this has ever been accomplished, but we 
find many regions in which erosion has almost reached base level. 
Such a region is called a peneplain (almost a plain). Peneplains 
present a very even sky line, broken only by occasional masses of 
the harder rocks that have resisted erosion. These masses some- 



PLAINS AND PLATEAUS 363 

times rise high above the general level, and are then called monad- 
nocks. Fig. 192 shows the uplifted peneplain of New England 
and Mount Monadnock, which is taken as the type of such relict 
mountains. There are several monadnocks in New England be- 
sides the one which bears the name, and there are others in Geor- 
gia and elsewhere. 

The mantle rock of a peneplain may consist of sediments depos- 




Fig. ig2. — The Uplifted Peneplain of New England 
Note that the ridges are of uniform height and that the sky line is straight. 

ited in former lakes and rivers, or of drift deposited by the conti- 
nental glacier; but these local deposits do not indicate the true 
history of the region. It is the inclination of the strata of bed 
rock that shows the size of the folds removed and the extent of 
the work of erosion, and it is the erosion of the bed rock, rather 
than differences in the thickness of the mantle rock, which gives 
the region its plain-like characteristics. 

Glacial Plains. — In some regions a continental glacier spreads 
till or bowlder clay over large areas of somewhat irregular bed 
rock, producing level lands like the well-known till plain of north- 
western Ohio. Fig. 193. 



364 ' PHYSIOGRAPHY 

Water from melting ice carries rock waste from the front of the 
glacier and deposits it in imperfectly assorted layers, forming an 
" out- wash plain " in front of a continental glacier, and " valley 
plain " in front of a valley glacier. 

The Great Western Plains extend from the Gulf of Mexico to 
the Arctic, and from the Mississippi River to the Rocky Mountains. 




Fig. 193. — Till Plain Near Columbus, Ohio 

The mantle rock here is unstratified glacial drift (till) spread smoothly over somewhat uneven 

bed rock. Many prairies are of this origin. Photographed by Professor E. Orton. 

On the west they reach an altitude of about 6,000 feet, but their 
width is so great and the rise so uniform that the eye does not de- 
tect it. 

The Great Plains are not so smooth as most plains formed by 
deposition ; they have been corraded by streams to some extent, 
but some areas are smooth, and when their great extent is con- 
sidered, the irregularities become insignificant. 

The mantle rock of the region, in some sections was deposited 
by ancient rivers, in other sections it was deposited by modern 
rivers, and in still others it resulted from the decay of the bed 
rock. 

The bed rock is not everywhere parallel to the surface of the 
plain; it has been tilted and bent since it was deposited as a sedi- 



PLAINS AND PLATEAUS 



365 




366 PHYSIOGRAPHY 

ment, and afterwards eroded until the slope of the surface is nearly 
uniform from the mountains to the prairies on the east. It is this 
fact that shows that the region is a worn down plain. 

Economic Importance of Plains. — Plains are the great agricul- 
tural regions of the world. The soil is fertile and well watered as 
a rule, and means of communication, such as canals, roads, and 
railroads, are more easily constructed and maintained than on 
uneven lands. These conditions are most favorable for agricul- 
ture. Such regions are always developed more rapidly than 
either plateaus or mountains. 

The desert regions of the World are often plains. They are 
deserts in some cases because they are arid, in others because 
they are frozen. In the United States many thousand acres of 
land formerly included in the " Great American Desert " are now 
under cultivation, which is made possible by irrigation and by 
the recently developed process of dry farming. The tundras, 
or frozen plains of Alaska, Canada, and Asia, are of course unfavor- 
able for agriculture. 

PLATEAUS 

Definition. — Both plains and plateaus are regions of broad, 
relatively smooth upper surface and usually horizontal bed rock. 
Although plateaus are, as a rule, higher than plains, it is not pos- 
sible to distinguish between them on the basis of altitude. The 
Piedmont Plateau, between the Appalachian Mountains and the 
Atlantic Coastal Plain, is much lower than the plains of the 
Mississippi Valley ; and the Appalachian Plateau has an altitude of 
2,500 to 5,000 feet, whereas the Great Plains east of the Rocky 
Mountains reach an altitude of 6,000 feet. The only possible 
distinction seems to be based upon the relative altitude of the 
plateau and the surrounding regions. 

A plateau is a region of broad summit area that is conspicuously 
higher than adjoining land or water on at least one side. 

Comparison with Plains. — The steep slope of the plateau front 
gives to plateaus certain distinct characteristics. The rivers 



PLAINS AND PLATEAUS 



367 



which flow from the plateaus to the adjoining lowlands are swift 
and, other things being equal, have greater corrading power than 
those of plains. This enables them to form deep valleys or 
canons and to establish a drainage system which will dissect 





Fig. 195. — Faults in Stratified Glacial Deposits, Rochester, N. Y. 



the plateau. On the other hand, the rivers of plains are 
without this steep slope, and therefore corrade less rapidly, giving 
greater permanence to the level surface of the plain. It should not 
be inferred from the above statement that the greater rate of 
corrasion of the plateau stream would reduce the plateau to base 
level before the neighboring lowland reaches it; there is more rock 
to be corraded in the plateau than in the plain, and as the plateau 



'368 



PHYSIOGRAPHY 



is eroded the streams gradually lose their steepness, and have no 
greater power of corrasion than those of the plain. 

How Plateaus are Formed. — Plateaus may be formed by the 
elevation of the region along a fault plane, by the depression of 
the adjacent country, and by lava flows. The lava plateau of 
Oregon and Idaho (Fig. 194) is an illustration of the last process. 
It is probable that our plateaus and mountains reached their 
present altitudes through many slight changes of level, rather than 




Fig. 196. — A Fault Plateau 

a single mighty uplift. Since erosion began its work of wearing 
down the region as soon as it appeared above the sea level, it is 
evident that the present altitude of a plateau or of a mountain 
simply shows to what extent the uplifting forces have outstripped 
the wearing down forces. 

Fault Plateaus. — The plateaus of northern Arizona, cut by the 
Colorado River, consist of a series of broad level areas, each one 
of which is separated from the next by a steep cliff, giving the 
region the appearance of a giant stairway. One of the cliffs, 
Hurricane Ledge, is 1,800 feet high. Such plateaus are formed 
by breaking the bed rock and displacing the rock on one side of 
the break; they are sometimes called Fault Plateaus. A break in 
rock, along which one side has been elevated or depressed, is called 



PLAINS AND PLATEAUS 



369 




Fig. ig7. — A Fault Plane 

The rock on the right of the fault is "Ruin Granite," that on the left is quartzite, and the 
more rapid erosion of the granite has exposed the fault plain. 

a fault (Fig. 195), and the cliffs which separate the "steps" of a 
broken plateau are called fault cliffs. Fault cliffs do not retain the 
slope of the fault plane. They are quickly eroded, so that their 
slope gives no indication of the angle of the fault; but the position 
of the cliff does indicate the location of the fault. 'Fig. 196. 

Life History of Plateaus. — The changes produced by weathering 
and corrasion are of the same nature in a mountainous region or 



370 PHYSIOGRAPHY 

on a plateau as on a plain, and the terms youth, maturity, and old 
age are employed to designate the stages in the life history of 
each. The principal difference is in the rate at which the 
changes are accomplished. The streams of a plateau front 
have steep slope, and therefore corrade their beds more rapidly 
than the same volume of water would wear away a plain. 
The great velocity also tends to keep the stream straight, thus 
minimizing the work of widening the valley by lateral corra- 
sion. Weathering, the chief method by which the valleys are 
widened, on a young plateau always fails to keep pace with the 
downward corrasion; hence, in youth, the drainage courses of 
plateaus are almost always canons or gorges. 

A Young Plateau has a comparatively smooth upper surface, 
often cut to great depth by swift streams, forming canons and 
narrow valleys. Fig. 199. The plateau cut by the Grand Canon 
of the Colorado is young. Fig. 194. 

As maturity is approached, the velocity of the streams dimin- 
ishes, because they have cut down toward the base level of the 
region, and weathering gains on corrasion, transforming the ca- 
nons into V-shaped valleys. During this process numerous trib- 
utaries develop, and the flat, upper surface becomes a system of 
ridges separating the valleys. 

The Appalachian Plateau, which extends along the western border 
of the Appalachian Mountains from the Hudson River to Georgia, 
is mature. The evidence of its once continuous upland surface lies 
in the fact that the tops of its numerous ridges form a nearly level 
sky line, and that the horizontal strata exactly match on oppo- 
site sides of the valleys. Fig. 198. 

The altitude of this plateau on the east is 2,500 to 5,000 feet, 
which is greater than that of the mountain ridges east of it. 

Roads, railroads, and towns are situated in the valleys. This 
region is one of abundant rainfall, and this, with the steep slope, 
has given the streams exceptional corrading power, thus develop- 
ing one of the most perfect illustrations of a mature plateau to 
be found in this country. Fig. 200 shows a cross section of this 
region drawn to scale. The numerous streams are subject to de- 



PLAINS AND PLATEAUS 



371 




Fig. ig8. — Tut Appalachian Plateau in West Virginia 

Note that the hilltops form a straight sky line, indicating that the surface was smooth before 

the valleys were corroded. 

structive floods and are loaded with rock waste. They are sepa- 
rated by narrow ridges, often 1,000 feet high, and so steep as to 
render agriculture impracticable. The nearly horizontal layers 
of rock are exposed in many valleys and have revealed the valu- 
able mineral resources of the region — iron ore, coal, petroleum 
and natural gas. The region is well forested, and lumbering is an 
important industry. The only cities in the region owe their 




Fig. iog.— Diagram of a Young Plateau 



372 



PHYSIOGRAPHY 



growth to the development of these resources. Chariest o wn, W. Va, 
is an important city illustrating this fact. It is a center from 
which much coal and petroleum is shipped. 

Old Plateaus. — If a plateau should be completely reduced to 
base level it would become a plain, showing no evidence of the 
existence of the plateau. Many worn down plateaus show evi- 
dence of their former altitudes in the remnants of the higher, 







f? 


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Fig. 200. — Cross Section of the Appalachian Plateau, near Charlestown, W. Va. 
Scale, i inch equals two miles. 

more resistant layers which have been preserved. In New Mexico 
there are a number of elevated areas that have been preserved, 
either because of the durability of the upper layer of rock, or 
because of their location with respect to drainage lines, and which 
show that many hundreds of feet of rock have been removed and 




Base /eve/ 



Fig. 201. — Diagram of an Old Plateau 
Showing a Butte and a Mesa. 



that the region was formerly a plateau. These flat topped areas 
are called mesas, if large with nearly vertical sides, and buttes if 
small. Figs. 202, 203 and 204. 

Economic Importance of Plateaus. — High plateaus are colder 
and usually more arid than the adjoining lowland. In tropical 
regions this is an advantage, and the upland is usually an im- 
portant agricultural region. For example, the plateau of Mex- 



PLAINS AND PLATEAUS 



373 




Fig. 202. — Antelope Butte 
These are monuments which show that much rock has been eroded. 




The upper layer is gypsum, 30 feet thick. 



374 PHYSIOGRAPHY 

ico furnishes the northern grains to a region where semi-tropical 
products abound on the lowland. In temperate climates the low 
temperature is a disadvantage. In the plateau of Thibet the great 
elevation causes the climate to be almost arctic, and much of the 
region is abandoned to wild animals and tribes of nomads. The 
centers of the settled and agricultural population of Thibet lie in 




Fig. 204. — A Smaller Butte 

the south. Some of the deeper valleys here are fertile and warm 
enough to produce two crops a year. ' 

Arid Plateaus. — The depth of the river valleys, even in moist 
plateaus, tends to lower the level of the ground water, thus in- 
creasing the difficulty of getting water. In regions of limited rain- 
fall, therefore, plateaus are less suited to agriculture than plains. 

Some farms flourish on our arid southwestern plateaus near the 
mountains, where mountain streams may be used for irrigation, 
and some other sections are fair grazing lands ; but as a whole the 
region is unoccupied, just as is that of Thibet. 



PLAINS AND PLATEAUS 375 

QUESTIONS 

1. What spherical bodies owe their shape to gravitation? 

2. Do solids assume a spheroidal form when rotated? 

3. In what state must the earth have been when its spheroidal form 
was assumed? 

4. Does this support the Nebular or the Planetesimal Hypothesis? 

5. What benefit does man derive from the relief of the earth? 

6. Why does the finding of the skeleton of a whale in glacial gravels 
near Lake Champlain indicate a change of level of the land in that 
region? 

7. What sort of evidence would show that a given plain was of lacus- 
trine rather than of marine origin? 

8. Fig. 192 shows a dissected region with hilltops forming a straight 
sky line. What would determine whether it was an uplifted peneplain 
or a dissected plateau? 

9. How was the original upland surface made level? 

10. Are all lacustrine plains fertile? Why? 

11. What facts prove that the Atlantic Coastal Plain was once a part 
of the continental shelf? 



CHAPTER XXV 
MOUNTAINS 

Peaks and Mountain Groups. — There is uniformity neither in 
structure nor in mode of formation of the isolated peaks and 
groups of elevations to which the term mountain is popularly 
applied. Vesuvius and Etna are popularly called mountains, 
although they are merely heaps of erupted materials about the 
vents from which they issued. Their formation was due to the 
action of internal forces which did not distort the bed rock. 

The Henry Mountains, in southern Utah, consist of a group of 
domes formed by the intrusion of lava beneath horizontal layers 
of bed rock, which were lifted to a great height and have since 
been eroded. They were formed by the action of internal forces 
which distorted the upper strata of bed rock. 

The Uintah Mountains, of Utah and Wyoming, consist of a 
broad fold or ridge, in the formation of which several thousand 
feet of sedimentary rock were uplifted and faulted. Their forma- 
tion is due to the action of internal forces which folded and faulted 
the bed rock. 

Lookout Mountain is a remnant of an old plateau, and the 
Catskill Mountains of New York are a part of the dissected plateau 
described in the last chapter. 

Each owes its form to the action of external forces, which eroded 
the surrounding bed rock without displacing its nearly horizontal 
strata. Each of the peaks mentioned above "mounts toward the 
sky," and is a conspicuous feature of the landscape. Each con- 
forms with the popular definition of a mountain given in the 
dictionaries. A mountain is a conspicuous elevation of limited sum- 
mit area. It is manifestly impossible to formulate a scientific 
definition of the term mountain based upon structure or mode of 



MOUNTAINS 377 

formation. Mountains cannot be distinguished from hills on the 
basis of absolute altitude. The Alleghany Mountains are much 
lower than the Black Hills, but they are the most conspicuous 
elevations in their section; whereas the Black Hills are "dwarfed 
by the Rocky Mountains." 

Some mountains, like Mt. Etna and Pike's Peak, consist of a 
single sharp summit or peak; others consist of long ridges. 

A mountain ridge is a mountain having much greater length 
than breadth. 

A mountain range is a ridge, or group of parallel ridges, formed by 
the same mountain-making effort. They are formed by the action 
of great internal crushing forces which fold or fault and tilt the 
bed rock, and are always regions of disordered strata. All of the 
ranges of the world are alike in these particulars, and for this rea- 
son it was formerly customary to say that mountain ranges thus 




Fig. 205. — Cross Section of Fault Mountains 

formed were the only true mountains. There are two types of 
mountain ranges, the folded range, in which the bed rock is com- 
pressed into folds, and the fault mountains, in which the bed rock 
was faulted and tilted. 

A mountain chain is a group of approximately parallel ranges 
formed by different mountain-making efforts. 

The term cordillera is applied to groups of mountain chains and 
ranges; for example, the cordillera of the western United States 
includes the Rocky Mountain chain, the Sierra Nevadas, and the 
Coast Range. 

Fault Mountains. — In the Great Basin region there are many 
ranges of very simple structure. They resemble fault plateaus, 
except that the bed rock has been tilted as well as faulted. The 
slope on one side is steep, that on the other side is gentle, 



378 



PHYSIOGRAPHY 




still too steep to permit the mass to be called a plateau. The 
strata of bed rock are parallel to each other and to the gentler 
slope of the block. Fig. 205. 

In southern Oregon are many such mountains, that were formed 
so recently that there has been time for little corrasion by the 
streams. It may be that they are still in process of formation, 
as earthquakes are still frequent in the region. 



MOUNTAINS 379 

Some of the ranges here are forty miles long, and in some places 
they rise 2,000 feet above the valleys. There are few settlers in 
this region, and their ranches are located in the stream valleys. 
The region is arid, and many of the lakes are salt. 

There are older fault mountains in Nevada and Utah which are 
very much dissected. In some instances these blocks are eighty 
miles long and twenty miles wide, rising from 2,000 to 7,000 feet 
above the valley. The crests are notched and uneven, and the 
slopes much corraded, forming sharp spurs and deep valleys. 
Fig. 206. 

The loftiest range in this country, the Sierra Nevada, is an 
uplifted fault block, but the faulting and tilting occurred in a 
mountain region after the strata had been folded and compressed. 
A great fault, some 400 miles long, formed along the western bor- 
der of the Great Basin, and a great mountain was uplifted and 
tilted downward toward the west. The steep slope of the block 
thus displaced is on the eastern side, facing the Great Basin. On 
the west the gentle slope leads down to the valley of California. 

On the eastern side of the Great Basin the Wasatch and the 
Teton ranges are fault mountains, their steep slopes facing west. 

The Sierra Nevada and the Wasatch Mountains are older than 
those of Oregon, and have been much dissected. 

Fault Mountains seem to have been formed by enormous 
lateral pressure, which faulted and tilted large blocks of the 
earth's crust. 

Folded Mountains.— The great mountain systems of the world 
are of this type. In most cases sedimentary rocks, formed in 




Fig. 207. — Cross Section of the Jura Mountains 

nearly horizontal layers on an ancient sea bottohi, have been 
crushed together and folded so as to form mountain ranges. 

One of the best examples of folded mountains is the Jura Moun- 
tains, between France and Switzerland. They consist of a series 



3 8o 



PHYSIOGRAPHY 




Fig. 208. — An Anticline near Hancock, Md. 




Fig. 209. — A Syncline in Shale, Upton, Pa. 



MOUNTAINS 



38i 



of parallel ridges. The rocks forming them are sedimentary and 
contain marine fossils; they were, therefore, formed on the sea 
bottom and were originally nearly horizontal. A cross section 
shows that they are now bent so that the layers are parallel to 
the mountain slopes, except where they have been eroded. 

As shown in Fig. 207, each ridge consists of layers of rock which 
form an arch. Such an upward fold or arch is called an anticline. 



isoo-p-JZ 




SOO—pitcsX 



Fig. 210. — Cross Section of the Appalachian Mountains, near Ellendale, Pa. 



Fig. 208. Each valley consists of a downward fold of the same 
layers. A downward fold or inverted arch, in a series of layers of 
rock, is called a syncline. Figs. 209 and 210. 

The Jura Mountains have been only slightly modified by ero- 
sion. The upper layers of the folds have been removed and the 
valley floors covered with rock waste; but the drainage of the 
region is controlled by the form of the mountain. Small streams 
flow down the steep sides and enter the main stream at approxi- 
mately right angles. 

Appalachian Mountains. — These mountains consist of folds like 
those of the Jura, but the folds are on a larger scale and are more 
complex. Many great faults are found, and the throw, or vertical 
displacement of one side of the fault in some cases is several 
thousand feet. The mountains are eroded so that the original 
summit lines have entirely disappeared, and the present ridges are 



382 PHYSIOGRAPHY 

the outcrops of resistant rocks which have withstood the action 
of the weather. 

Fig. 210 shows a cross section of the Appalachian Mountains 
near Harrisburg, Pennsylvania. The rocks under Third Mountain 
form a syncline which shows that the top of this mountain was 
once the bottom of a valley. If the anticline which corresponded 
to the Third Mountain syncline should be restored, we should 
have a mountain of several times the height of those now occupy- 
ing the region. The dotted lines suggest the probable height. 
It will be noted that the four ridges have about the same altitude, 
and the picture, Plate II, shows that the sky line formed by their 
summits is practically a straight line. This indicates that at some 
time in the past the mountains which occupied this region were 
reduced to a peneplain, because there is no other way in which 
erosion can produce an even surface. 

Rocky Mountains. — When the early explorers first saw these 
mountains in the distance, they reported the existence of vast 
ranges which glistened in the sun as though composed of crystals. 
Closer observation led others to call them the Stony Mountains, 
which name was changed to the more correct term of Rocky 
Mountains. Their striking feature is shown by these attempts to 
name them. They are great masses of bare and often crystalline 
rock, many peaks in Colorado reaching an altitude of between 
14,000 and 15,000 feet. The base is covered by a cloak of coarse 
waste washed down by the mountain torrents, and here the slopes 
are forested; but the timber thins out and disappears t about 
11,500 feet above the sea level. The irregular line which marks 
the upper limit of trees is known as the timber line. Above this 
line the talus slopes are small, and the bare rock cliffs and peaks 
above them are the characteristic features of the scenery. 

West of the Front Range, near Denver, Col., are other nearly 
parallel ranges of similar character, which approach each other 
quite closely in places and again bend away, leaving broad parks. 
These parks are in some instances 50 miles wide, and have a com- 
paratively level floor of rock waste washed down from the moun- 
tains. South Park, just west of Colorado Springs, is an example. 



MOUNTAINS 



&3 




384 PHYSIOGRAPHY 

It has an area of more than 1,000 square miles, and an altitude of 
about 8,000 feet. This is between 2,000 and 3,000 feet higher than 
that of the Great Plains east of the Front Range. South Park is 
drained by the Platte River, which flows down the eastern slope of 
the Front Range. 

Structure. — The central mass of the Rocky Mountains is gran- 
ite, or granitic rocks, but at the base, resting upon the granite, 
sedimentary rocks are found. These are tilted, and may be traced 
some distance up the slopes on either side. In some of the ranges 
these rocks undoubtedly once extended entirely over the surface 
and have been eroded. Other ranges may have been islands in 
the sea in which the sedimentary rocks were deposited, and there- 
fore received no sediments. In either case it is evident that the 
uplift occurred after the sedimentary rocks were formed, because 
they are tilted and are now many thousands of feet above sea level. 
Examination of these sedimentary rocks shows that they were 
deposited long after those of the Appalachian folds. The sedi- 
mentary rocks of the Appalachian plateau dip downward toward 
the Mississippi Valley, and are covered with rocks thousands of 
feet thick made from sediments which accumulated after the 
Appalachian Mountains were formed; and these upper rocks are 
involved in the folds of the Rockies. It is thus proved that the 
Rockies are younger mountains than the Appalachians. 

This conclusion is also reached from a comparison of the 
amount of work that erosion has already accomplished with the 
amount it may still accomplish in the two chains. 

When the earth's crust was folded to form the Rocky Moun- 
tains, the greater thickness of the layers of rocks involved required 
greater lateral force than was required to form the Appalachians; 
and this greater force produced a greater uplift, and at the same 
time fractured and faulted the rocks so that there were extensive 
outpourings of lava over the region. This accounts for the many 
igneous dikes and lava flows found there. 

This region, after having been nearly base leveled, or reduced to 
a peneplain on which some of the harder areas formed monadnocks, 
was uplifted. This revived the streams at the edge of the moun- 



MOUNTAINS 



385 



tains and produced many of the gorges, as the Royal Gorge of the 
Arkansas. 

Origin of Mountains. — If a metal ball could be heated or cooled 
uniformly throughout its mass it would retain a spherical form. 
To do this would require perfect conductivity of heat; but a poor 
conductor like the earth cools more rapidly at the surface than 
in the interior, and therefore contracts unequally. 

Both the Nebular and the Planetesimal Hypotheses assume that 
the earth has long had a solid and highly heated interior, and they 
both agree that the crust of the earth, or the lithosphere, as it is 
often called, has probably been at approximately the present 
temperature for millions of years. During these years the interior 
has been losing heat and contracting. 

The generally accepted theory of the origin of mountains is based 
upon these assumptions. This theory maintains that the interior 
of the earth has contracted materially since the lithosphere reached its 
present temperature and size, causing it to wrinkle because it is too 
large for its shrunken interior. 

In mountain ranges there is evidence of enormous lateral pres- 
sure, which folded and compressed strata once horizontal. It is 
easily shown that the contraction of the interior of the earth, 
after the lithosphere had reached a permanent size, would cause 
lateral pressure. In Fig. 212, if 
L represents the lithosphere, and 
C represents the shrunken in- 
terior, the action of gravity on a u ^- 
section of the lithosphere, S, 
would be like driving a wedge 
into the lithosphere, and would 
compress it laterally. Every 
other section would do the same, 
and this action explains the ex- 
istence of a certain amount of 
compression, but does not satis- 
factorily account for all of it. 

Fig. 212.— Diagram of Gravity and 
There have been many changes of Lateral Pressure 




386 PHYSIOGRAPHY 

temperature in the strata of the lithosphere, and every rise of tem- 
perature must have added to the lateral pressure, due to gravity, 
through the expansion of the heated layers. Lava flows and intru- 
sions have heated adjoining rock, expanding and adding to the gravi- 
tational pressure. Rock strata are being constantly buried beneath 
the sediments and every layer deposited above a given layer tends 
to raise its temperature and to produce lateral pressure. This action 
occurs chiefly on the borders of the continents. 

There are undoubtedly radio-active substances in the bed rock which, 
like radium, slowly emit heat. Such substances tend to raise the 
temperature of the rocks containing them, producing further expansion 
and corresponding increase in lateral pressure. Such temperature 
changes have undoubtedly developed local lateral pressure on a large 
scale. 

Erosion in Mountains. — All of the lofty mountain ranges show 
evidences of erosion on a grand scale. Their peaks and horns 
have been carved out of the solid rock of wavelike folds and 
rounded domes; their ledges and cliffs have been profoundly 
riven by frost and changes of temperature, and their slopes 
scarred and gullied by mountain torrents and glaciers. 

The following conditions which prevail in mountain regions explain 
the effectiveness of the agents of erosion: 

First, their altitude usually leads to heavy rainfall or snowfall, on at 
least one side, which means in either case rapid erosion on the side or 
sides receiving the precipitation. Moreover, their altitude subjects 
them to greater daily range of temperature and the increased rate of 
weathering due to this extreme variation. If the fluctuations of temper- 
ature center about the freezing point of water, the rate of weathering is 
further increased by the alternate freezing and thawing of water in the 
crevices and pores of the rock. Fig. 93 shows how much the solid 
rock of Pike's Peak has been broken by this action. 

Second, their steepness gives to their streams and glaciers a velocity 
and power of corrasion which exceeds that possessed by the streams of 
plains or by continental glaciers, and rock waste is removed. 

Their steepness also enables gravity to remove rock waste from the 
face of the cliffs and ledges, thus constantly exposing fresh rock to the 
action of the weather. For the same reason the creep of the rock waste 
down the slope of the mountain is exceptionally rapid. 

These various agents change land forms, which without their action 
would have been bounded by smooth curves and uniform slopes, into 
crags, needles, and peaks. Many such were named Sierras by the 



MOUNTAINS 387 

Spaniards because they resembled a saw. Fig. 211 of the La Plata 
Mountains shows this effect. 

The first effect of erosion, then, is to increase the strength of the 
relief of the mountain region. The irregular surface of the slope of 
the dissected ranges of Utah, Fig. 206, illustrates this action. With 
the lapse of time the peaks are worn down and rock waste covers 
the whole mountain, which once more consists of rounded domes 
and ridges. In this stage they are known as subdued mountains. 
Fig. 214. As time passes the region approaches more and more 
nearly to the peneplain. The more resistant rocks wear down 
more slowly than the weaker, and often stand up conspicuously 
above the peneplain. Mount Monadnock, in southern New 
Hampshire, is a typical illustration. 

As soon as erosion begins to round the mountain form, it begins to 
decrease the relief of the region, and continues to decrease it until 
the close of the cycle of erosion. The present height of all moun- 
tains depends quite as much upon the vigor of the agents of erosion, 
the time which these forces have been in action, and the ability of the 
rocks to resist erosion, as upon the magnitude of the uplifting forces 
and the time during which the action of the uplifting forces con- 
tinued. 

Life History of Mountains. — We have seen that mountains are 
acted upon by two sets of forces, the one tending to make them 
higher, the other tending to make them lower; the first acts from 
within the earth, the second from without. 

Mountains have their period of growth and their period of decline. 
Growth lasts as long as the uplifting forces are more effective 
than the agents of erosion, and decline begins when the rate of 
erosion exceeds the rate of uplift, and continues until the uplifting 
forces renew their activity and raise the region more rapidly than 
it is eroded, or until the region becomes a peneplain. 

Erosion begins as soon as the region is raised above the sea, and 
continues as long as any portion of the region remains above sea 
level. The uplifting forces are not necessarily continuous in their 
action, but may be intermittent, ceasing entirely for a time. 
They usually are more active during the early history of a range 



3 88 



PHYSIOGRAPHY 



and perhaps stop permanently, in some cases, when the earth's 
crust becomes strong enough to resist their action. There is evi- 
dence that after the Appalachian Mountains were worn down to 
a peneplain, the uplifting forces acquired renewed vigor, and that 
the region had more than one period of uplift. 




Fig. 213. — The Jtjngfrau, a Young Mountain 
Note the U-shaped glacial valley in the foreground showing former extension of Alpine glaciers 



During the warfare of constructive and destructive forces, the 
characteristics of mountains change so that one may readily dis- 
tinguish young mountains from old ones. 

The Rocky Mountains are younger than the Appalachians, and 
differ markedly from them. For example: The Rockies have bare 
ledges and cliffs, with small talus slopes; the Appalachian rocks 
are usually waste covered. The Rockies have a very irregular sky 
line ; the Appalachians present an even sky line. The Rockies rise 
8,000 or 9,000 feet above the platform on which they rest; the 
Appalachians 900 to 1,200 feet. Avalanches and landslides occur 
occasionally in the Rockies but not in the Appalachians. The 
streams in the Rockies are young; those in the Appalachians are 
more mature. 



MOUNTAINS 



389 



Young Mountains, Figs. 211 and 213, are characterized by ir- 
regular sky line, bare ledges, steep slopes, streams in the torrential 
stage, and the summit lines still in their original positions. During 
youth, avalanches, landslides and earthquakes occur at times. 

Subdued Mountains, illustrated in Fig. 214, have uniform slopes, 
low, rounded form, and few bare ledges; earthquakes or avalanches 




14. — Subdued Mountains in North Carolina 
Compare the broad rounded summits with those in Fig. 211. 

are rare or unknown ; the sky line is more regular than that of young 
mountains, but less regular than that of old mountains; water gaps 
and passes have developed. 

Old Mountains. — As mountains grow old and the region ap- 
proaches a peneplain, monadnocks stand out here and there with 
uniform and forested slopes of deep rock waste. The original 
summit lines have disappeared and new ones have developed, 
following the outcrops of more durable rocks. The peneplain of 
New England, Fig. 192, and the region south of Lake Superior 
are old mountain regions. 



390 PHYSIOGRAPHY 

The uplift and decline of mountains takes place so slowly that 
the change produced during a lifetime passes unnoticed, and men 
have come to think and speak of them as everlasting. History 
fails to give us assistance in determining the rates at which these 
changes progress. Polybius' description of the Alps as they were 
when Hannibal crossed them in 218 B.C. is practically a descrip- 
tion of the Alps to-day. The Alps are still young mountains, and 
the lapse of 2,000 years has not materially changed them. It is 
evident from these considerations that the life history of moun- 
tains cannot be expressed in terms of years or even in thousands 
of years, and that the time required to wear down our old moun- 
tains to the present peneplains was very long. 

Climate of Mountains. — The snow-capped mountains of the 
Torrid Zone exemplify upon their slopes all the climatic changes 
that one would experience in traveling from the Torrid Zone to 
the Polar regions. As one ascends, the palms and bananas of the 
Torrid Zone gradually disappear, and are replaced by the decidu- 
ous trees and wild flowers of the Temperate Zone. These in turn 
are replaced by the cone-bearing trees, which, as the ascent is con- 
tinued, become low and dwarfed; finally all trees disappear. 
Above this point grasses and bright Alpine flowers flourish; but 
these also disappear as the ascent continues, and the snow-clad 
top is a Frigid Zone in miniature. In a similar manner the forms of 
animal life that inhabit the bases of such mountains gradually dis- 
appear, and are replaced by forms which characterize the higher 
latitudes. 

The great variety in mountain climate is due to the fact that 
the vertical temperature gradient in air at rest is more than 1,000 
times as great as the average horizontal temperature gradient. 
That is to say, the average annual temperature decreases more 
than 1,000 times as fast as one ascends as when one travels pole- 
ward. 

Numerous observations both in balloons and on mountains 
have established the fact that the average rate at which the tem- 
perature falls as we ascend is one degree Fahrenheit for every 300 
feet. 



MOUNTAINS 391 

The timber line and snow line are more or less irregular, being 
usually higher on the south or sunny side of east and west ranges 
than on the shady side. In the equatorial region the snow line is 
about 18,000 feet above sea level, but its altitude diminishes as 
the distance from the equator increases, reaching sea level in the 
Arctic and Antarctic regions. 

Many ranges are subject to excessive rainfall or snowfall on 
the windward side; and where they cross prevailing winds the 
climates of the opposite slopes are in sharp contrast. For example, 
on the western slope of the Sierra Nevadas, the moist wind is 
chilled as it rises, producing abundant rainfall, which supports 
forests; whereas the same wind on the eastern slope, having lost 
most of its moisture and being heated by compression as it de- 
scends, becomes a drying wind which takes moisture from the 
land, making it arid. A similar distribution of rainfall occurs in 
the Cascade Mountains, but the region east of them is less arid 
than that east of the Sierras, because the Cascades are lower. The 
heavier rainfall is also found on the west slopes of the Rockies 
and Andes, in the belts of the prevailing westerlies, and on the 
east slope of the Andes in the trade wind belts. 

The south side of the Himalayas has much heavier rainfall than 
the north side in the summer, because they lie across the path of 
the southwest monsoon. As a rule, however, the contrast between 
the sides of ranges parallel to the prevailing wind of a region is less 
than that between the sides of ranges crossing the paths of the 
prevailing winds. 

Mountain ranges sometimes deflect winds, changing their direc- 
tion and bringing rain to regions that would not receive it under 
other conditions. 

Habitability of Mountains. — The difficulty of crossing moun- 
tains, the danger from avalanches and landslides, the low tem- 
perature of the summits, and the great cost of transportation, 
combine to make mountain regions less desirable for habitation 
than plains or plateaus. 

If the difficulty of making a living is overcome, many features 
of mountains attract men to them. The healthfulness, the gran- 



392 PHYSIOGRAPHY 

deur of the scenery, and the military advantages of mountains have 
led many to make their homes among them. 

Influence on Man and History. — Because of the difficulty of 
crossing mountain ranges, the difference in climate on the different 
sides, and the military advantages which they afford, mountain 
ranges are the natural boundary lines for nations. The Himalayas, 
which separate different races; the low Pyrenees, crossed by but 
a few roads and railroads; the Caucasus, the Alps, and the Andes, 
all illustrate the tendency of nations to select mountain ranges for 
their frontiers. 

As the Indian and the pioneer gained a measure of security 
within their stockades, so a nation surrounded by mountain ram- 
parts is in a measure secure from outside interference. It requires 
a greater incentive to cause outside nations to attack them than 
is required to lead them to attack nations not so surrounded. The 
elevation of their outposts enables them to see an approaching 
enemy that would be invisible on a plain, thus diminishing the 
chance of surprise. Narrow passes well fortified can be success- 
fully defended against vastly superior numbers, because the in- 
vading army cannot approach the pass in line of battle and is 
met in small parties. The famous defence of Thermopylae illus- 
trates this advantage. 

The soldier on the mountain meets a tired foe, and in hand-to- 
hand conflict this is an important aid. Artificial avalanches of 
bowlders have frequently decimated armies attempting to cross 
mountain passes. When Hannibal crossed the Alps his losses 
through this kind of warfare contributed in no small measure to 
his ultimate defeat. 

Because of the security afforded, conquered races usually make 
their last stand in mountains, and have frequently been able to 
maintain their position through long periods, some of which ex- 
tend even to the present day, as the Basques, the Welsh, the 
Highlanders, etc. 

With the military advantage comes a degree of isolation which 
favors the development of a distinct type of civilization and an indi- 
vidual language, or dialect, in the region thus set apart from the 



MOUNTAINS 393 

rest of the world. This tendency is illustrated in the many small 
principalities which developed in Europe during the Middle Ages, 
several of which exist to-day; and in the fact that in the Cali- 
fornia valleys there were almost as many tribes of Indians having 
characteristic languages and customs as there were valleys be- 
tween the mountains. 

The same isolation limits commerce and knowledge of the out- 
side world, and compels the residents of mountainous regions to 
depend upon themselves for their wares and for their progress. 
If their number is small, as it is apt to be on mountain slopes, 
where the struggle for existence is so strenuous, there is rarely 
progress in the ways of civilization, but instead there is often a 
retrograde movement. Mountaineers are proverbially conserva- 
tive, using the same processes and following the same customs 
that their forebears used and followed. In the southern Appala- 
chians we find excellent illustrations of this effect; here are peo- 
ples following habits and customs of the eighteenth century. 
Mining cities in mountains are exceptions. To them the sudden 
wealth brings all that is good and all that is bad in our modern 
civilization. 

Mountain ranges retard the exploration and settlement of a region. 
The outfit which an explorer must carry is heavy. If he follows 
the rivers, shelter and food for many weeks can be transported in 
a canoe by one man; but if he journeys over plains the number 
of men and wagons increases rapidly as the proposed journey is 
lengthened; and if he is to cross mountains pack animals must 
replace wagons, without further increase in the size of the party. 

There is no better illustration of this retarding action than that 
found in the early history of this country. Before the year 1600, 
European explorers had visited the mouths of the St. Lawrence, 
the James, the Mississippi, and the Rio Grande, and had visited 
California. During the next century the English explored and 
settled the Atlantic coastal plain, but made few attempts to cross 
the low ridges of the Appalachians; the French, during the same 
period, explored the St. Lawrence and followed the Mississippi 
to the Gulf. They established settlements along the routes which 



394 PHYSIOGRAPHY 

grew into towns still bearing French names, such as Detroit, 
Sault Ste. Marie, Fond du Lac, Prairie du Chien, St. Louis, and 
Baton Rouge. The Spanish settlers on the Gulf of Mexico, during 
the sixteenth and seventeenth centuries, extended their missions 
toward the north as far as Santa Fe, where the Rocky Mountains 
checked further progress in this direction. They therefore pushed 
westward to southern California. From there they followed the 
Pacific coast toward the north, establishing missions in the narrow 
area between the Coast Range and the Pacific. Their trail is now 
marked by cities still having Spanish names, such as San Antonio, 
Sante Fe, and along the coast, San Diego, Los Angeles, San 
Francisco, and Sacramento. 

The Berkshire Hills, in Massachusetts, exerted an important 
influence in settling the contest between Boston and New York 
City for commercial supremacy. Freight brought from the West 
through the Mohawk Valley to Albany could be brought to New 
York by boat more cheaply than it could be hauled over the Berk- 
shires by teams, and much of it was naturally deflected to New 
York. When railroads were built along the Hudson and through 
the Mohawk Valley, New York City acquired further advantage 
over Boston because of the Berkshires. Before a railroad line from 
Albany to Boston was completed, the position of New York as 
the chief seaport of the United States was fully established. 

Mountains are not absolute barriers. They are difficult to cross, 
but when sufficient incentive is provided, men always succeed in 
crossing them. 

In the case of the English colonists the necessary incentive came 
in the demand for more room and more virgin soil, and in the 
increased importance of the trans-Appalachian fur trade. During 
the French and Indian War which followed, the possession of the 
best passes through the mountains was stubbornly contested, as 
is shown by the large number of battlefields between the Hudson 
and Lake Champlain, and between the Mohawk and Lake On- 
tario. 

The Rocky Mountains retarded the settlement of California 
more effectively than the Appalachians confined the colonists to 



MOUNTAINS 395 

the Atlantic coast, and for a longer period, because of their greater 
height and breadth; but the necessary incentive came in the dis- 
covery of gold in 1848. Before the close of 1849, there were 100,000 
people in California. 

Barriers to Plants and Animals. — As the white man appeared 
first on the eastern shore of North America and gradually spread 
westward, so it is probable that each species of both animal and 
plant life appeared first in some definite locality and gradually 
spread from this center. Man is the only form of life that is capa- 
ble of adapting itself to all conditions of altitude and climate, 
and is therefore the only species of life that has spread over an 
entire continent. Various physical features act as barriers which 
certain forms of life cannot cross, and among them, perhaps, the 
long mountain range is as effective as any feature. No physical 
feature is equally effective as a barrier to all species of animal and 
plant life. Mountain ranges, which have checked the spread of 
the white man for long periods, are but slight obstacles to the 
spread of birds. 

The low temperature of the summit of mountains prevents 
certain forms of animal life from crossing them. The spread of 
some species of animals is checked by mountains because of the 
steepness of the mountain slopes; and still other species are pre- 
vented from crossing by predatory animals inhabiting higher 
altitudes. It is said that the Asiatic ranges limit the spread of 
even the mountain goat to such an extent that every range in the 
region has developed a distinct species. 

The climate of high mountains prevents the spread of plants. 
Above the timber line no trees grow, even though their seed 
reach the region; and a short distance above the timber line no 
form of vegetation can develop. Certain winged seeds and those 
like the seeds of the thistle and the milkweed, may be blown over 
mountain ranges, and some are undoubtedly carried over by birds; 
but, as a rule, the native plants, like the native animals inhabit- 
ing the opposite sides of long mountain ranges, are of different 
species. 



396 PHYSIOGRAPHY 

Economic Value of Mountains. — i. Mining. — The forces which 
formed our mountain ranges subjected the rocks of the regions to 
greater stress than the horizontal rocks of the plains and plateaus 
underwent, and the resulting fractures and faults are more numer- 
ous and are less uniform in shape than those of other regions. 
Each fracture in impervious rock becomes a channel through 
which underground water may circulate, and in which veins of 
various minerals may be formed by the waters. 

Other features which facilitate the formation of mineral veins 
in mountain regions are the heavy rainfall, which increases the 
volume of ground water; the elevation of the region above the 
surrounding country, which gives the ground water circulating 
through the underground passages an increased " head " and in- 
creases the rate of flow; and sometimes higher underground tem- 
perature, due to intrusion of igneous rocks or other causes. 

These four conditions — the greater number of underground 
passages, the greater volume of circulating water, the greater 
liquid pressure of the water, and the higher temperature of the 
water — account in a measure for the fact that most of the mines 
of ores that occur in veins are found in mountains. 

In Fig. 215 it will be seen that all of the gold and silver mines 
of the country are in either the western mountain ranges or in the 
Appalachian region. The great number of gold mines on the 
western slope of the Sierra Nevada Mountains, where they have 
the maximum height and the maximum rainfall, cannot be acci- 
dental. Most of the copper mines and many of the lead and zinc 
mines are similarly located. Important copper mines are located 
among the worn down mountains near Lake Superior. 

Coal, iron, and salt occur in beds rather than veins, and these 
are not chiefly found in mountains. See map of distribution of 
coal, page 253. 

Mountain-making processes have metamorphosed many rocks, 
and some of them are mined or quarried in mountains. For exam- 
ple, anthracite coal comes chiefly from Appalachian mines, and 
much slate and marble are quarried in the Green Mountains. It 
is not only the more frequent occurrence of valuable ores and rocks 



MOUNTAINS 



397 




398 PHYSIOGRAPHY 

in mountains that makes mining the most important industry there; 
this is also due to the fact that erosion has revealed the structure 
and deposits of the region, thus making the discovery of mineral 
deposits a simpler problem there than eleswhere. Mines are oper- 
ated in young as well as old mountains. 

2. Water Power. — The water power of mountain streams has 
long been utilized in cities along the fall line, and by the miner in 
western mountains, but only a small percentage of mountain 
streams can be thus utilized. The possibility of electric trans- 
mission of power has greatly increased the value of these streams, 
and the public interest in the "white coal," as water power is 
called, speaks for its rapid development. It is destined soon to 
become a second important industry in the mountain regions, 
and a great stimulus to our manufactures. 

3. Agriculture.— -The mountain slopes are obviously unsuited for 
agriculture; the soil is usually poor and its cultivation laborious. 
The grasses above the timber line, to be sure, furnish pasturage 
during the summer, and occasional less steep slopes allow the 
farmer to raise the necessary food; but his life is one of poverty 
and hardship until the coming of the summer visitor transforms 
him into a guide or a hotel keeper. 

The valleys in mountain ranges are often fertile, and when well 
watered make valuable farms, but the farmer here is handicapped 
by the difficulties of transportation. He must haul his surplus 
products and his supplies over the mountain ridges which sur- 
round him. 

4. Irrigation. — Although many mountain ranges cause arid re- 
gions on their leeward side, they also make it possible to restore 
fertility through irrigation. The United States Government is 
building many reservoirs in the mountain valleys of the West, 
where streams or canals may lead water from them to level regions 
during the growing season. Without these reservoirs most of the 
stream beds in the arid regions would be dry for most of the 
year. 

5. Timber Reserves. — The growing scarcity of lumber has called 
attention to the fact that mountain slopes make excellent timber 



MOUNTAINS 



399 







reserves, and our Government has already set apart many square 
miles of them for this purpose. They are patrolled to prevent the 
destruction of growing timber by fires, and, properly guarded, will 
do much to supply the future generations with lumber. 

Timber reserves act to some extent as do the reservoirs for irri- 
gation, in that they conserve the rainfall and tend to make the 
streams more permanent. 

The Geographic Cycle. 
— The major relief fea- 
tures of the land, such as 
continents, mountains, 
and plateaus, are acted 
upon by two sets of forces 
which in a way oppose 
each other. The construc- 
tive forces originate below 
the surface of the earth 
and depend upon the in- 
ternal heat of the earth 
for their activity; and the 
destructive forces, or the 
agents of erosion, originate 
in the atmosphere and 
depend upon the heat of 
the sun for their activity. 










The agents of erosion Fig. 216.— Irrigation Centers of the West 

o<-t , lnrin Q ll lonrl tlrif ic The black portions show the land to be irrigated by the 
act Upon ail lana mat IS works the Government has built or is now building. 

above sea level, and only 

cease to act when the land disappears beneath the sea. Ero- 
sion is continuous in its action, whereas the constructive forces 
are irregular and often intermittent, causing the upbuilding to 
cease for a time. Although at the beginning the rate of upbuild- 
ing is much more rapid than that of erosion, the slower but con- 
tinuous action of the destructive forces ultimately prevails. Every 
relief feature, therefore, has a period of growth, during which the 
constructive forces accomplish more work than the destructive 



400 PHYSIOGRAPHY 

forces, and its period of decline, during which the agents of erosion 
prevail. 

During the growing period the feature is said to be young; after 
erosion is well established and has developed so many hills and 
valleys that they become the chief characteristics of the region, 
it is said to be mature; finally, when both the constructive and 
the destructive forces have nearly ceased to act upon the region, 
it is said to be in its old age. 

When land is first lifted from the sea the sedimentary strata are 
horizontal and the surface is a plain from which other physical 
features may be developed. The final condition of all physio- 
graphic features is a level surface, a peneplain, at or just below sea 
level. This is a return to the level surface from which the relief 
feature was formed; and the time required for the upbuilding and 
destruction of a relief feature is properly called a geographic cycle. 
The description of the various stages through which a relief feature 
passes constitutes the life history of the land form. 

There is no -positive evidence that any geographic cycle has 
ever been completed, but in many regions the constructive forces 
have remained inactive long enough for erosion to reduce the 
region to a peneplain before the activity of the constructive forces 
was resumed and a second geographic cycle begun. That the 
Appalachian Mountains were worn down to a peneplain and then 
uplifted is proved by the facts that many anticlinal ridges have 
disappeared and that in their places are many ridges with nearly 
level tops. Where erosion has once made a surface uneven, the 
only way in which it may again become level is by reduction to a 
peneplain. When a second cycle is established in such a region, 
the rivers begin their work with graded beds and quickly cut young 
valleys in them, thus recording the evidence of a second cycle. A 
cycle may be lengthened by elevation of the region, or shortened 
by depression, and the interruption may occur at any stage. 

The length of a geographic cycle varies greatly in the case of 
the different features. Our great mountain ranges and plateaus 
have changed so slightly during historic time that it is evident 
that tens or even hundreds of thousands of years may be required 



MOUNTAINS 401 

to complete their cycle. At the other extreme are the volcanic 
islands which sometimes have been raised from the ocean in a 
few days, only to disappear again beneath the sea in a short time. 
It is obvious that in a given case the length of the cycle depends 
upon: (a) the total uplift; (b) the energy and the character of the 
eroding agents; and (c) the resistance of the rocks. The total up- 
lift is not the "initial uplift" sometimes mentioned in this con- 
nection, because land forms are not made by a single effort, but 
grow gradually through many small uplifts. It is greater than the 
greatest height of the land form, because erosion is active during 
the period of growth. The energy of the agents of erosion depends 
upon the slope of the streams, the amount of precipitation, the 
strength of the winds and the variability of the temperature; or 
briefly upon steepness of the region and its climate. The resist- 
ance of the rocks to erosion depends upon the physical properties 
of the rock, such as brittleness, porosity and hardness, upon the 
chemical composition, upon the strength of the cement which con- 
solidated the rock, and upon the structure. With so many variable 
factors there must naturally be great variation in the length of the 
cycles. 

QUESTIONS 

1. Why is "Rocky Mountains" a more correct term than the name 
"Stony Mountains," first given them? What is a stone? 

2. Show how a syncline may become a hill. 

3. In Fig. 205 a wedge-shaped block has dropped down. Does this 
indicate that the force that formed the faults on either side of it was a 
thrust (compressing the rock) or a pull (stretching)? 

4. What kinds of rock form the summits of the ridges shewn in Fig. 
210 (Appalachian Mountains)? Why? 

5. What do you think would have been the effect upon the settlement 
of the United States if a continuous mountain range of great height had 
been where the Appalachians now are? 

6. Can you account for the presence of Arctic plants on the tops of 
isolated high mountains near the tropics? 

7. Why is the western slope of the Sierra Nevada Range forest covered, 
whereas the eastern slope resembles the Great Basin in, barrenness? 

8. Mention several conditions which would tend to make the geo- 
graphic cycle exceptionally long. 



CHAPTER XXVI 
VOLCANOES AND EARTHQUAKES 

Volcanoes have long been objects of interest to students of his- 
tory and mythology, and much has been written of the Mediter- 
ranean group even by early Greek and Latin authors. This same 
region is also classic ground from a physiographic standpoint, 
because of the careful scientific study that has been made here 
of the phenomena of an eruption. 

Definitions. — A volcano is an opening in the earth through 
which lava and other heated materials are ejected. Some of the 
ejected materials pile up around the opening and form a cone of 
greater or less steepness, as the material forming it is coarse or 
fine, liquid or solid. In the top of the cone there is usually a 
cup-shaped depression called a crater. 

Causes of Volcanic Action. — Some geologists maintain that the 
heat comes from the interior of the earth; others that it is pro- 
duced near the surface by chemical or mechanical means. A later 
theory of the origin of the heat maintains that it is due to the 
action of radio-active substances like radium, which are known to 
be present in lavas. Whatever the origin of the heat, the fact 
remains that volcanoes are associated with young and growing 
mountains, and this suggests some relation between the uplifting 
forces and the origin of the heat. 

The force which causes explosive eruptions is undoubtedly 
steam pressure. All lavas contain water in greater or less quan- 
tity, and when they are deep down in the earth the pressure keeps 
the water in the liquid state; but as the lava ascends, the pressure 
diminishes, until at last the water suddenly becomes steam. If 
the lava is very fluid, the steam rises quietly, unless it is confined, 
and oozing eruptions are likely to result. If the lava is confined 



VOLCANOES AND EARTHQUAKES 



403 



by any means, the pressure increases as more and more water 
changes to steam, and finally the covering rock bursts, just as a 
steam boiler does when the pressure becomes too great. 

Phenomena of Eruptions. — From the preceding examples it will 
be seen that the phenomena of an ordinary explosive eruption 




Fig. 217. — Cotopaxi, Ecuador. Symmetrical Cone (Stlbel) 

occur about as follows: A mighty explosion blows off the top of 
the cone, shatters the hardened lava, and sends steam, mingled 
with dust and ashes, high into the air, where it spreads out as a 
peculiar " cauliflower cloud." The falling stones and ashes destroy 
vegetation and may even bury whole cities. The rising steam, 
cooled by expansion and by mingling with the cold upper air, is 
condensed and falls as rain, accompanied by lightning. The rain 
brings down dust and ashes, and all together form immense mud 
torrents, capable of burying cities, as for example Herculaneum. 
In volcanoes of the type of Vesuvius, after the explosion the 
liquid lava rises in the crater until it either overflows or, more 
frequently, until by its great pressure it rends the mountain and 
a lava flow escapes through the fissure thus formed. At first the 




Fig. 218. — Chimborazo. Domelike Cone (Stubel) 



404 



PHYSIOGRAPHY 



lava may flow rapidly, but it gradually cools, hardens, slackens in 
speed, and finally stops. 

Columnar Structure. — The lava that cools and slowly solidifies 
under great pressure contracts more or less symmetrically around 
a central core, breaking into columns. This characteristic colum 




Fig. 219. — Ruminahui, with Great Side Crater (Stubel) 



nar structure is exemplified in the Palisades of the Hudson, in 
Fingall's Cave, Giant's Causeway, and at Regla. 

History of the Cone. — A volcanic cone, like all land forms, 
passes through a cycle of growth and decline, and we easily recog- 
nize the stages of youth, maturity and old age. 

In youth, the cone retains its symmetry unobscured by erosion. 
It is little dissected, though it may be scarred by stream and 
glacial valleys, as Mount Shasta in northern California. 

In maturity, the destructive forces of erosion have reduced the 
cone to an unsymmetrical and dissected mass. 

In old age, erosion has so destroyed the cone that only the core 
of the volcano remains. The volcanic ash and cinders have 
been for the most part removed and there remains, standing out 
prominently, the plug of hardened lava that once filled the vent. 
This plug, known as a volcanic neck, is the last remnant of the cone 




Fig. 220. — Puluxagua, Ecuador. Caldera, with Inner Cone (Stubel) 



VOLCANOES AND EARTHQUAKES 



405 




406 



PHYSIOGRAPHY 



to disappear. Mount Royal, which gives its name to Montreal, 
is an example of a volcanic neck. 

Distribution of Volcanoes. — Active volcanoes are found where 
the crust is weakest, that is in or near the sea and in young and 
growing mountains. Most volcanoes lie in one of two belts. The 
best marked belt surrounds the Pacific Ocean. The other belt is 
an irregular one, passing through the Hawaiian Islands, the Medi- 
terranean Sea region, and intersecting the first belt in the East 
Indies and in the West Indies. 

Products of Volcanic Eruptions. — At the instant that they are 
ejected, nearly all of the products are in either the liquid or the 
gaseous state, the only exception being the rock fragments torn 
from the sides of the crater or blown from the overlying rocks. 
With the cooling of the products the steam condenses to water 
and only the sulphur dioxide, carbon dioxide, hydrogen sulphide, 




Fig. 222. — Columnar Structure of Cooled Lavas 
Regla, Mexico. (Kindness of Prof. J. F. Kemp.) 



VOLCANOES AND EARTHQUAKES 



407 



chlorine and related substances remain as gases. Lava that is 
free from bubbles of gas and solidifies quickly forms obsidian. 

When the lava is projected high into the air and solidifies be- 
fore reaching the earth, it may be so filled with bubbles of the 
expanding gases contained in it that it becomes frothy or spongy 
and is called pumice. 

If the ejected materials cool before falling to the ground, they 
are known by various names, depending upon the size of the 
particles: volcanic dust, ashes, lapilli, and bombs. Materials of 
all sizes up to " the size of an ox " are ejected. Volcanic bombs, 
if their contained gases have expanded them, as does bread in 
baking, are called bread cake bombs. 

Economic Products. — A Scotch firm purchased the cone of Vul- 
cano, a small Mediterranean volcano, because of the alum, boracic 
acid, and sulphur that could be obtained from it. 

Pumice, sulphur, and borax are important volcanic products. 

Trap rock was used to pave the streets of Rome and the famous 
Appian Way; similar blocks from old volcanoes in Germany are 
floated down the Rhine to face the dykes of Holland; and from 
the Palisades comes much of the material to pave the streets and 
parks of New York City. Volcanic dust and ashes when consoli- 
dated form tuff, a soft stone easy to work in the quarry, but 
hardening in air and becoming a very durable building stone, 
much used in Naples and Rome. Some of the oldest sewers in 
Rome, built of tuff 2,500 years ago, are still in good condition. 

Volcanic dust and ashes exposed to plentiful rainfall rapidly 
weather and form a very fertile soil. Some of the finest orchards 
of New Jersey, some of the best farms of Oregon, and some of the 
most fruitful vineyards of Germany are on soils of volcanic origin. 




Fig. 223.— I, Laccolite. II, Intrusions with Dikes. Ill, Extrusion with Dike. 
IV, Vent for Ashes, etc. (Penck) 



408 PHYSIOGRAPHY 

Stromboli. — An irregular cone of volcanic material rises from 
the floor of the Mediterranean some 36 miles north of Sicily, to a 
height of about 6,000 feet, one-half of which is above sea level. 
For more than 2,000 years this volcano has been in a state of 
mild activity, emitting clouds of steam and showers of stones, 
and at night illuminating the cloud, which usually hangs over it, 
with flashes of light. This "Lighthouse of the Mediterranean" 
has guided sailors for centuries. Light, curling columns of steam 
rise from fissures in the crater at all times. At intervals, without 
the slightest warning, a sound is heard "like that produced when 
a locomotive blows off steam," and a great volume of watery 
vapor, carrying many small masses of lava, is thrown violently 
into the air. The lava bombs are often in a semi-molten condition 
when they fall to the earth. Such outbursts occur at frequent 
intervals, and are due to the escape of great bubbles of steam 
through the chilled and tenacious surface of the molten lava that 
fills the cracks. 

Etna. — The giant cone of Etna was known to the Romans as the 
" Forge of Vulcan." It is two miles high and about forty 
miles in diameter at its base. There are some 200 minor cones 
on its slopes. Its eruptions are preceded by earthquakes and loud 
explosions. Smoke, ashes, and cinders are discharged, and finally 
lava flows from the new cone formed. The large proportion of 
lava accounts for the gentle slope of the cone of Etna. 

Vesuvius. — The ancients knew Vesuvius as a mountain rather 
than as a volcano. At the beginning of the Christian era its crater, 
then about three miles in diameter, was covered with vegetation. 
Its slopes were cultivated and towns were located at its base; and 
there was no record of previous volcanic activity of the mountain. 

During the summer of the year 79 a.d., a series of earthquakes 
of increasing severity occurred, and a new and strange cloud 
formed above its summit. Explosion after explosion occurred 
within the mountain and the black cloud spread, shutting out the 
light of the sun. Tacitus gives us two letters from the younger 
Pliny, who was an eyewitness of this eruption. One of these letters 
describes the experiences of his uncle, the elder Pliny, who lost his 



VOLCANOES AND EARTHQUAKES 



409 



life near the foot of Vesuvius during an eruption. It seems that 
his party sought shelter from the shower of cinders and stones in 
a villa which " shook from side to side " from frequent earth- 
quakes. When the accumulation of stones and ashes made it 




Fig. 224. — Explosive Eruption of Vesuvius in 1900 
Steam with ashes, cinders, and bombs. (Matteucci.) 



apparent that the villa would be buried, the party took to the 
fields, " with pillows tied about their heads with napkins " to 
protect them from the falling stones. 

The second letter relates the younger Pliny's experiences at 
Misenum, across the Bay of Naples from Vesuvius. , He describes 
chariots standing on level ground without horses, which would 
not stand still even when the wheels were blocked with great 



410 PHYSIOGRAPHY 

stones, but "kept running backward and forward" with each 
earthquake; and says, "Besides this, we saw the sea sucked 
down and, as it were, driven back again by the earthquake." 
Across the bay above Vesuvius " was a dark and dreadful cloud, 
which was broken by zigzag and rapidly vibrating flashes of fire, 
and, yawning, showed long shapes of flame. These were like light- 
nings, only of greater extent. ... Soon the cloud began to de- 




Fig. 225. — Eruption of Vesuvius in April, 1906 
As seen from Portici. 

scend over the earth and cover the sea. . . . Ashes now fell, yet 
still in small amount. I looked back. A thick mist was close at 
our heels, which followed us, spreading over the country like an 
inundation. . . . Hardly had we sat down when night was upon 
us — not such a night as when there is no moon and clouds cover 
the sky, but such darkness as one finds in close-shut rooms. . . . 
Little by little it grew light again. We did not think it the light 
of day, but proof that fire was coming nearer. It was indeed 
fire, but it stopped afar off; and again a rain of ashes, abundant 
and heavy; and again we rose and shook them off, else we had 



VOLCANOES AND EARTHQUAKES 411 

been covered and even crushed by the weight. . . . Soon the real 
daylight appeared; the sun shone out, of a lurid hue, to be sure, 
as in an eclipse. The whole world which met our frightened eyes 
was transformed. It was covered with ashes white as snow." 

No lava flow accompanied this eruption, but the enormous 
quantity of ash buried Pompeii and, mixed with rain, formed a 
mud stream which overwhelmed Herculaneum. There have been 
frequent eruptions of Vesuvius since this one, those of 163 1 and 




Fig. 226. — Idealized Profile of Vesuvius 

A, prehistoric ashes of Monte Somma; B, lava flows of Vesuvius; C. ash cone of Vesuvius; 

D, parasitic cones; E, molten lava of interior. (Penck.) 

1906 being especially destructive. In these later eruptions the 
explosive action has been followed by lava flows. 

The eruptions of Vesuvius are unlike the mild, continuous action 
at Stromboli, and consist of paroxysms of great violence separated 
by long intervals of quiet. During these intervals of rest the vol- 
cano is said to be dormant. 

Mont Pelee.— An eruption of this volcano on May 8, 1902, de- 
stroyed the city of St. Pierre on the island of Martinique, one 
of the Lesser Antilles. Previous to this date it had been dormant 
for fifty years, but for days before the eruption it had shown signs 
of activity. Great columns of steam and ash were ejected, boiling 
mud flowed from the sides of the volcano, and repeated explosions 
occurred in its interior. Lightning flashed from the ascending 
cloud, and the frequent earthquakes broke all ocean cables lead- 
ing to the island. 

On the morning of May 8th, a dull red reflection was seen on 
the trade-wind cloud that covered the mountain summit. This 
became brighter and brighter, and soon red-hot stones were 
ejected from the crater and bowled down the mountain sides, 



412 PHYSIOGRAPHY 

giving off glowing sparks. Suddenly a hot blast of gases shot 
from the crater, and two minutes later engulfed the city of St. 
Pierre, five miles distant, in an atmosphere that was fatal to all 
who breathed it. It wiped out all vegetation and every living 
creature in its path. Buildings of the city and ships in the harbor 
instantly burst into flames; 30,000 persons lost their lives. There 
was no lava flow, but the lava in the throat solidified and was 
forced upward by the pressure from below until it stood 1,200 
feet above the crater at its maximum. 

Krakatoa. — In 1883 the most violent explosive eruption of his- 
toric times occurred on the East Indian island of Krakatoa. The 
island was some five miles long and three miles wide, with an alti- 
tude of 2,623 feet at its highest point. 

Nearly the whole of the lower part of the island and half of the 
highest peak were blown away. Dust was thrown into the air to 
a height of about twenty miles, and was carried several times 
around the earth. Beautiful sunrise and sunset effects were caused 
for many months by this dust. The concussion of the explosion 
broke windows in Batavia, 100 miles away, and the report was 
heard 2,267 miles. A mighty wave flooded the surrounding coasts 
to a depth of fifty feet, stranding ocean steamers, causing great 
loss of property, and drowning more than 36,000 people. For 
many weeks navigation was impeded by floating pumice that 
covered the surface of the sea. 

Hawaiian Islands.— Hawaii is one of a group of islands in which 
are many volcanoes, and which in the main owe their existence to 
eruptions at the bottom of the ocean. This island is 80 miles long, 
and rises 30,000 feet above the ocean floor. There are four craters 
on the island, of which Mauna Loa is the highest. The eruptions 
of the volcanoes in the Hawaiian Islands are in sharp contrast 
with that of the island of Krakatoa. In these oozing eruptions 
there are no explosions, no showers of dust or ash, and no great 
volume of steam is ejected, and earthquakes are rare. The lava 
flows sometimes continue for months, whereas eruptions of the 
explosive type last but a few days. Before an eruption the lava 
rises quietly in the crater until the great pressure fissures the side 



VOLCANOES AND EARTHQUAKES 413 

of the mountain, when a river of molten rock flows to the sea. 
The slopes of the volcano subject to oozing eruptions are very 
gentle, but this must not be understood to mean that the cone 
is small. Mauna Loa is many times as large as Vesuvius, and its 
crater is a typical caldera, nearly three miles long, two miles wide, 
and 1,000 feet deep. Icelandic volcanoes are of this type. Caldera 




Fig. 227. — Cinder Buttes in Idaho (Penck) 

is a Spanish word meaning caldron. The term is applied to large 
craters believed to have been formed by the sinking of the top of a 
volcanic mountain. 

Volcanoes of North America. — Active volcanoes are numerous 
in Central America and Mexico, and some of the Alaskan vol- 
canoes have been in eruption within a few years; but we have no 
reliable description of an eruption within the limits of the United 
States proper. There are, however, many evidences of former 
great volcanic activity. 

San Francisco Mountain. — This mountain in Arizona is much 
eroded and no signs of a crater remain, but it is surrounded by 
lava flows and beds of cinders, and several hundred cinder cones, 
formed by volcanic eruptions, are found in the immediate vicinity. 
Some of these cones were formed so recently that erosion has not 
modified the original form of the cone. (Fig. 227.) 



414 PHYSIOGRAPHY 

Mount Taylor. — On one of the large mesas of western New 
Mexico, Mount Taylor, rises to an altitude of n,ooo feet. The 
mountain is almost entirely composed of lava, and the mesa is 
covered by a cap of lava. This cone is also much eroded, and in 
the lowland about the mesa are many volcanic necks, each one a 
mass of lava which cooled in the throat of a volcano that has dis- 
appeared. 

Mount Shasta. — This extinct volcano of northern California is 
in some respects like Etna. It towers n,ooo feet above a base 
seventeen miles in diameter, is snow clad even in summer, and 
its eruptions were explosive, followed by great lava flows. There 
are two great craters, the younger being near the top of one side 
of the older cone. Some twenty smaller cones are found near the 
base of the mountain, and from one of these a lava flow may be 
followed more than fifty miles. The cone is much dissected by 
glaciers and streams, but is still in its youth. 

Mount Hood, on the crest of the Cascade Range in Oregon, is 
noted for its graceful outlines and for the fumaroles and steaming 
rifts which still emit sulphurous fumes and indicate compara- 
tively recent activity, although there has been no eruption within 
the memory of man. 

Mount Rainier. — This stately cone rises from near sea level to 
an altitude of 14,500 feet, and so appears much higher than most 
of those that reach a greater altitude. It has a bowl-shaped 
crater, below which on the sides of the mountain the rims of 
former craters may be seen. Jets of steam and gas still issue from 
small holes in its snow-clad summit, showing that its heat has not 
entirely disappeared. 

Other Indications of Volcanic Activity. — The Columbian lava 
plateau covers a large part of Washington, Oregon, and Idaho 
with successive layers of lava, which in places reach a total thick- 
ness of 5,000 feet. The section of this plateau suggests stratified 
rock, but each layer represents a distinct flow of lava, and is 
sometimes separated from the next by layers of soil in which the 
roots and trunks of large trees are preserved. This proves that a 
long interval of time elapsed between the flows. Because of the 



VOLCANOES AND EARTHQUAKES 



415 




41 6 PHYSIOGRAPHY 

absence of cones in this region, it is thought that the lava came 
through fissures. The surface is covered with residual soil of 
great fertility. This plateau is cut by many deep canons in which 
the structure of the plateau is shown. 

There are few indications of volcanic activity east of the Rocky 
Mountains. Erosion has destroyed such volcanic cones as may 
have existed, but it has exposed numerous sheets of lava which 
were intruded between the layers of sedimentary rocks. The 
Palisades of the Hudson, the lava sheets of the Connecticut Val- 
ley, and the Watchung Mountains of New Jersey are examples of 
such sheets. 

EARTHQUAKES 

Definition. — The lithosphere is constantly in a state of tremor. 
Sometimes the vibrations are so slight as to pass unnoticed ex- 
cept as recorded by the most sensitive instruments, and at other 
times so violent as to destroy whole cities. Some of the tremors 
are due to human activities, such as the movement of railway 
trains or the explosion of dynamite; others are due to natural 
causes. 

The vibrations travel through the lithosphere and may be 
detected at greater or less distances from their source, as the vio- 
lence of the shock which causes them is greater or less. 

An earthquake is a tremor of a part of the lithosphere produced by 
natural causes. 

The Ischian Earthquake. — On July 24, 1883, the island of 
Ischia, near Naples, Italy, was shaken by an earthquake which 
was not preceded by warning shocks, and which lasted but 15 
seconds. Violent detonations accompanied the tremors, 1,200 
houses were destroyed, 2,300 persons were killed, fissures were 
opened, and landslips occurred. Survivors tell us that the whole 
town seemed " to jump into the air " and fall in ruins. 

On this island is the great crater of Epomeo, which was in 
eruption in 1302, after at least 1,000 years of slumber. No erup- 
tion of Epomeo accompanied this earthquake, but it is believed 
that the underground explosions which caused the earthquake 
were of volcanic origin and indicate future activity of Epomeo. 



VOLCANOES AND EARTHQUAKES 417 

The Charleston Earthquake. — During the last week of August, 
1886, slight earthquake shocks occurred at intervals at Charles- 
ton, S. C. Their violence gradually increased, and culminated 
at 10 p.m. on August 31, in one of the great earthquakes of the 
last century. There was first noticed a distant rumble, which 
increased in intensity as though an enormous railway train was 
approaching through a tunnel passing beneath the city. As this 
rumble became a roar the ground seemed to rise and fall in visible 
waves. The disturbance lasted about 70 seconds and was re- 
peated with equal violence eight minutes later. 

During these tremors men could not stand, chimneys were 
thrown down, and every building in the city was damaged. Great 
cracks were opened in the earth and both underground and sur- 
face drainage were disturbed; railroad tracks were twisted and 
bent, and 27 persons were killed. The shock was felt as far north 
as Canada. 

This earthquake is notable for the information concerning 
earthquakes derived from the study of its phenomena. The loca- 
tion of the origin of the disturbance was determined, and the 
velocity with which the earthquake wave traveled in this case 
was shown to be 150 miles per minute. 

The earthquake was succeeded by several less severe shocks 
during the night, and slight shocks were observed in the region 
for several months. 

The San Francisco Earthquake. — About 5 a.m., April 18, 1906, 
an earthquake occurred on the California coast which lasted 67 
seconds. During this short interval many buildings in San Fran- 
cisco were wrecked and the water supply was cut off, so that the 
fire which followed destroyed a large part of the city. Figs. 
230 and 231. Many landslides occurred at the same instant in 
the mountains of the district affected, cracks were opened in the 
earth, and some regions settled several feet. 

The earthquake was due to slipping along an old fault plane, 
which has been traced nearly 400 miles. Fig. 229. The 
average vertical displacement was slight, but the horizontal dis- 
placement was in places as much as 20 feet. 



4i8 



PHYSIOGRAPHY 



The Messina Earthquake. — At 5.23 a.m., December 28, 1908, 
the region about the Strait of Messina, in Southern Italy, ex- 
perienced one of the most disastrous earthquakes in the history 
of the world. The cities of Messina and Reggio were reduced to 
a shapeless mass of ruins, several smaller towns were more or less 




Fig. 229. — The Fault Trace. San Francisco Earthquake 



damaged, and upwards of 200,000 persons were instantly killed 
or imprisoned in the ruins, so that rescue was impossible. 

The ground seems to have been suddenly raised and then 
dropped, causing the buildings to collapse; great fissures opened; 
the wharf sank to the level of the sea, and a sea wave from six to 
ten feet high swept over the lower portions of the region. 

The earthquake was preceded by several slight shocks, and the 
seismic activity continued for several weeks. 

Extensive breaking of telegraph cables in the vicinity indicates 
a submarine disturbance, and the center of the disturbance was a 



VOLCANOES AND EARTHQUAKES 



419 




Fig. -'30. — Ruin of the $7,000,000 City Hall by the San Francisco Earthquake and Fire 




Fig. 231. — Mission Street, San Francisco, Af 



420 PHYSIOGRAPHY 

line through the Strait of Messina. These two facts make it 
probable that the earthquake was due to slipping along the old 
fault plane which runs through the Strait. This fault plane has 
probably been the seat of many earthquakes. The total vertical 
displacement of one side of this old fault is known to be several 
thousand feet. 

Distribution of Earthquakes. — No portion of the earth is en- 
tirely free from earthquakes, although most of them occur either 
in the vicinity of active volcanoes or near growing mountains. 

It will be observed that the borders of the Pacific Ocean are 
particularly subject to earthquakes, and that a belt of seismic 
activity crosses Eurasia, beginning at Gibraltar and following the 
general direction of the Mediterranean Sea. 

There have been in recent times relatively few earthquakes 
among the older mountains which border our eastern coast. 
Professor Shaler has called attention to the fact that in New Eng- 
land there can have been no violent earthquake since glacial times, 
for such an earthquake would have displaced the numerous bal- 
anced rocks to be found there. 

Cause of Earthquakes. — All great earthquakes are vibrations 
established by the sudden yielding of the earth's crust {the litho- 
sphere) to the stresses set up within it by lateral pressure. Such earth- 
quakes are steps in the natural process by which our plateaus and 
mountains have been uplifted. All of the earthquakes described 
above, except the Ischian, were of this class. In some instances 
the strains due to lateral pressure are relieved by fracture of the 
lithosphere, which is accompanied by faulting, and at others by 
lateral displacement of one side of the fissure, without faulting. 
In some instances the strains are relieved by slipping along an 
old fault plane. 

Many earthquakes are caused by explosions which accompany 
volcanic eruptions. The Ischian earthquake was probably of this 
type, and like others of its class was a minor disaster. 

Any natural phenomenon which results in a heavy blow to the 
lithosphere might throw a portion of it into a vibration which 



VOLCANOES AND EARTHQUAKES 



421 




422 PHYSIOGRAPHY 

would be classed as an earthquake, if the action occurred below 
the surface. Slight tremors may have been caused by great land- 
slides, by the fall of the roof of a large cave, or by similar acci- 
dents. 

Sea Waves. — When an earthquake occurs near the sea, great 
sea waves often increase the disaster. The water at first recedes 
from the land, sometimes leaving vessels stranded on the exposed 
sea bottom; this is followed by the advance of a great wave, 
which has in some instances swept the vessels over the tops of 
houses and has stranded them far inland. In the earthquake at 
Lisbon, Portugal, in 1755, some 30,000 people who had sought 
safety on the wharves were drowned by the sea wave. These 
waves have been called " tidal waves," an obvious misnomer. 

QUESTIONS 

1. Criticise the definition "A volcano is a burning mountain, belching 
forth fire, smoke and lava." 

2. How can people near a volcano tell that an eruption is probably 
about to take place? 

3. Contrast eruptions of Vesuvius, Pelee, and Hawaiian volcanoes. 

4. Give, in their natural sequence, the phenomena of an ordinary 
explosive eruption. 

5. State and account for the distribution of volcanoes. 

6. Which is the most valuable volcanic product? Why? 

7. What quakings of the earth are not properly called earthquakes? 
Why? 

8. Contrast the two great classes of earthquake phenomena. 

9. Define seismic and seismograph. 

10. State and account for the general distribution of earthquake 
regions. 

n. Why, after an earthquake near the seashore, does the water first 
recede? 

12. Where would there be more danger of earthquakes — along a moun- 
tainous, or along a coastal plain shore? Why? 



CHAPTER XXVII 

SHORE LINES AND HARBORS 

Definitions. — The shore line is the line along which land and 
water meet. The shore is the margin of land next to any large 
body of water, whereas the coast is the margin of land next to the 
sea. The beach is that portion of the shore that lies between high 
and low water levels. The continental shelf is the submerged por- 
tion of the continental mass adjoining the shore and extending 
seawards with gentle slopes. The depth of about 600 feet is gen- 
erally taken as the outer limit of the shelf. 

Migration of Shore Lines. — The fossil remains of sea shells and 
the character and arrangement of the rocks of the Atlantic Coastal 
Plain in the southeastern part of the United States indicate that 
in comparatively recent geologic time this plain was a continental 
shelf, and that the shore line of the Atlantic was near the inner 
border of the plain. This is shown in No. 1 of Fig. 233. 

The deep channels across the continental shelf opposite the 
mouths of the Hudson and other rivers indicate that what is 
now continental shelf was once a coastal plain, with its shore line 
near the outer border of the present continental shelf. This is 
shown in No. 2 of Fig. 233. 

Next the coastal plain was again almost entirely submerged, 
as is shown in No. 3 of Fig. 233. 

At present the shore line is between the two. No. 4, Fig. 233. 

These facts prove that the shore line has changed its position. 
Many other examples of migrations of shore lines could be given, 
to show that there have been in various parts of the world great 
transgressions of the sea over what had been dry land, and many 
exposures of sea bottom, changing it to land. 



424 PHYSIOGRAPHY 

Explanations of Migrations. — The cause of the migration of 
shore lines is probably the cooling and contracting of the crust of 
the earth; the resulting folds and breaks in the crust produce 
great blocks, the rising or sinking of which disturb the relative 
levels of sea and land. There are two explanations of how this 
disturbance is brought about. According to one explanation, the 
land rises and sinks with reference to an unchanging sea level. 
The other explanation, without excluding minor and local risings 
and sinkings of the coast, emphasizes the idea of a changing sea 
level. Thus a settling of a great portion of the bed of an ocean 
would withdraw the sea from its shores without any real rise of 
the land with reference to the center of the earth, although there 
might be an apparent rise of the land. On the other hand, great 
deposits of sediment in the sea, or the uplift of a portion of the 
bed of the sea, would cause the sea to overflow its borders with- 
out any sinking of the land. 

Classes of Shore Lines. — As the result of migrations, there are 
two great classes of shore lines— regular and irregular. 

The surface of the land is characteristically irregular; hence, 
when the sea is made to cover a portion of the land, causing the 
shore line to migrate landward, the shore line is irregular. Such 
shore lines are found along the northeastern and the north- 
western coasts of North America. 

The ocean floor is characteristically smooth as the result of 
long-continued deposition and of slowly moving water; hence, 
when the shore line migrates seaward, exposing smooth land, a 
regular shore line is formed. The western coast of the United 
States is an example. 

Waves, currents, and tides corrade weak rocks and sometimes 
make a shore line temporarily irregular; but they ultimately make 
the shore line regular by wearing away projecting portions of the 
coast, by filling bays, and by building sand reefs off shore. 

At many places between Long Island and the Rio Grande, 
shore line migration has resulted in an inner irregular shore 
line along . the mainland, in front of which the waves have 
built sand reefs forming an outer regular shore line facing the sea. 



SHORE LINES AND HARBORS 



425 




United States 
No. 1. Shore line at inner border of coastal plain. 
No. 2. Shore line next at outer border of continental shelf. 
No. 3. Shore line again near inner border of coastal plain. 
No. 4. Shore line at present. An. Rept. U. S. Geol. Survey, Vol. XI. 

Regular Shore Lines. — Where the shore line migrates seaward 
over a wide continental shelf, a coastal plain is formed with a 
regular shore line. Shallow water extends so far from shore that 
vessels run aground before danger is suspected. The islands are 
low sand reefs, built parallel to the shore by the waves, currents, 
and tides. Openings through the reefs are numerous where the 
tides are high. A few shallow harbors supply a hinterland devoted 
to agriculture, as in the southeastern States along the Atlantic 
and the Gulf of Mexico. 



426 PHYSIOGRAPHY 

Where the shore line migrates seaward, as the result of the rise 
of a mountain range, the shore line is generally regular. The coast 
is exposed to wave action and tends to become precipitous, with 
harbors few and unprotected. The western coast of the United 
States, Peru, and northern Chili are examples. 

Africa and India are plateaus, with shore lines that have re- 
mained regular since they were shaped by the great faults in the 
earth's crust that permitted bordering regions to sink under the 
sea. 

Kinds of Irregular Shore Lines. — When the shore line migrates 
landwards, so many bays are formed that the coast is called an 
embayed coast. An embayed mountain region differs so much 
from an embayed coastal plain that it is convenient to consider 
them separately. 

Pacific Type of Coast. — Ranges of mountains parallel the shores 
of the Pacific Ocean. From northwestern United States southward 
to central Chili the coast is regular, except for a few passages 
through the coast ranges into rather long bays paralleling the 
coast, San Francisco Bay and Puget Sound, for example. But in 
higher latitudes the shore line is irregular; this is, because along the 
northwest coast of North America and the southwest coast of 
South America the encroachment of the sea has changed some 
ranges of mountains into chains of high, rocky islands paralleling the 
mainland, and separated from it by tortuous inner passages. 
The eastern coast of Asia has wide bordering seas, separated from 
the Pacific Ocean by great festoons of islands paralleling the 
mainland. 

Ria Coast or Atlantic Type. — In northwestern Spain, in Nova 
Scotia and in Maine, the mountain ranges, instead of being 
parallel to the sea, advance to meet it at an angle and at the 
shore end abruptly, as if broken off. This enables the sea to 
enter the mountain valleys and to form long bays that merge 
into rivers. Such bays are called rias. 

The Fiord Coast. — This is found in glaciated regions, such as 
Norway and Labrador, where glaciers have modified the valleys. 



SHORE LINES AND HARBORS 



427 




428 PHYSIOGRAPHY 

Fiords are long, narrow, deep bays, with high precipitous sides, 
that were occupied by ice long enough for the glaciers to change 
the V-shaped valleys to U-shaped. Although the Maine coast 
has been called a fiord coast, its bays lack the length, depth, and 
high sides of the true fiord. 

Economic Importance. — The value of an embayed mountain 
region depends in part upon the products, the climate, and the 
accessibility of the mountainous hinterland; and in part upon 
the products of the sea. The harbors are good, and fishing and 
commerce thrive. The people are seafaring, self-reliant, and hardy. 

Embayed Coastal Plains. — When a shore line migrates landward, 
covering a coastal plain, stream valleys are drowned and become 
bays. Such an embayed coastal plain will have a more or less 
irregular shore line as the valleys were more or less numerous. 
As time elapses, sand reefs with regular outer shore lines may 
border the region. The currents along shore move the sand with 
them, extending capes in the direction of motion and reducing 
the capes facing the currents. Note the length of Cape May 
peninsula and the shortness of Cape Henlopen. 

Economic Importance. — Embayed coastal plains combine with 
the advantages of coastal plains the additional advantages of 
numerous harbors and of greater accessibility from the sea. This 
accessibility made it easy for the colonists to settle Virginia and 
to market their heavy crops of tobacco, and has made Baltimore 
an important port. 

The Dutch have built great dikes around large tracts of land 
covered with rather shallow water, and have by means of wind- 
mills pumped the water off, leaving polders. A large portion of 
the Zuyder Zee is being so reclaimed at great expense. Much 
tidal marsh land in and near New York City has been filled in, 
but thousands of acres more will be reclaimed. 

Coral Reefs — Southern Florida, the Hawaiian Islands, and 
the shores of all oceans of the Torrid Zone, except the eastern 
shores of the Atlantic and the Pacific, are fringed with jagged 
coral reefs. 



SHORE LINES AND HARBORS 429 

The reef-building coral is a small animal living in colonies at- 
tached to the ocean floor. It requires clear, warm, salt water 
currents to bring it food, and light, which it cannot get much 
below a depth of 120 feet. It extracts limestone from sea water 
and deposits it in the lower part of its body. By the growth and 
decay of countless corals, the rocky base may be built up nearly 
to the surface of the sea. The waves break off branches of the 
coral and grind them to coral sand which finally consolidates to a 
granular limestone. The waves and the wind may build up a 
low reef, not over 20 feet above the level of the sea. 

Where the reef is close to the shore, as along eastern equatorial 
Africa, Brazil, Cuba and the Hawaiian Islands, it is called a 
fringing reef. The outer border, better supplied with food, grows 
more rapidly than within. Within the corals die, and the rock 
is dissolved until a lagoon may develop inside the barrier reef, as 
it is now called. The Great Barrier Reef of the northeast coast 
of Australia is about 1,000 miles long. 

An atoll, or ring of coral around a central lagoon, may be formed 
where the coral has grown on the top of a shoal that comes to 
within 120 feet of the surface. Or, according to Darwin's theory, 
the atoll is a coral reef around a sunken island, as for example a 
volcano. The growth of the coral equalled the rate of sinking. 
When the rate of sinking, or rise of water, exceeds the rate of 
growth, the coral polyps are drowned. The Chagos Islands in 
the Indian Ocean are the unsubmerged portions of a very exten- 
sive coral region. 

Coral islands are naturally very low, though some few show 
that they have been elevated. Plant life may be abundant, 
though of few varieties. The cocoanut palm furnishes food, 
clothing, and utensils to the unambitious natives. 

HARBORS 

Historic Importance. — There has been a close relation between 
harbors, trade and the spread of civilization ever since the Phoeni- 
cians carried their own alphabet and the products of the civiliza- 



430 PHYSIOGRAPHY 

tions of both Egypt and Asia from Tyre and Sidon into Greece, 
Carthage, and the western Mediterranean world. 

Rome was the most convenient harbor on the borderland be- 
tween Greek civilization and Etruscan civilization in Italy. After 
she had destroyed the harbor of her great rival, Carthage, Rome 
became the mistress of the Mediterranean world. 

During the Middle Ages and the Renaissance, the products of 
the civilizations of the East and West were distributed largely 
by those cities that had good harbors, the Italian cities of Venice 
and Genoa, and the Hansa cities of Germany. 

In modern times Holland, Spain, England, the United States, 
and Germany have owed no small part of their rapid advance in 
power and wealth to their numerous good harbors. 

Requirements. — A harbor is a place affording anchorage and 
safety to shipping. A place from which vessels sail and to which 
they return is a port; if it has a custom house for the legal entry 
of merchandise, it is called a port of entry. A good harbor has the 
following requirements: a good entrance, good anchorage, room 
for many ships, freedom from ice in winter, good docking facilities, 
and a productive country tributary to it constituting its hinter- 
land. 

Classes of Harbors. — There are three great classes of natural 
harbors, with several varieties of each: river harbors, including 
deltas and estuaries; bay harbors, including fiords and craters; 
and lagoon harbors, both sand reef and coral. 

River Harbors. — Hamburg, in northwestern Germany on the 
Elbe, is and has long been one of the great ports of the world. 
There is easy anchorage for vessels of deep draught, and river, 
canal, and railroad transportation to all Germany as a hinter- 
land. Antwerp on the Scheld, in Belgium, although sixty miles 
from the sea, is one of the principal ports of the world. New 
Orleans, over ioo miles from the mouth of the Mississippi, with 
the central part of the United States as hinterland, is a delta 
port. One objection to some river ports is illustrated here — the 
ease with which a bar is formed at the mouth of the river, imped- 



SHORE LINES AND HARBORS 431 

ing navigation. This is prevented in the Mississippi by building 
jetties. 

Some estuary ports — Quebec, Canada, and Bristol, England, for 
example — have to contend with excessive tides. At Liverpool the 
bar that prevented the passage of large vessels except at high tide 
has been removed, and the difficulty of loading and unloading 
from vessels that are raised and lowered twenty feet by the tides, 
has been overcome by building an immense floating landing-stage 
with movable bridge approaches. 

Bay Harbors. — Bay harbors vary widely in size and importance. 
Along shore lines of the Pacific type they are few ; but when pres- 
ent they may be spacious, and within the inlet are more or less 
parallel to the outer shore line. Excellent examples are San Fran- 
cisco Bay, Puget Sound, with its several ports, and the Bay of 
Rio de Janeiro. 

The typical Norwegian fiord, although well protected, has steep, 
wall-like sides that render docking difficult. The hinterland is 
also poor and inaccessible. 

Crater harbors, as St. Thomas, West Indies, are deep and well 
protected. 

Lagoon Harbors. — Coral reef and sand reef harbors are better 
protected from waves than from winds which sweep over the low 
reefs. Both are generally shallow and difficult to enter. Through 
coral reefs the inlets are tortuous and bordered by jagged coral 
rocks. Key West, Florida, Pearl Harbor, in the Sandwich Islands, 
and the Island of Guam, are the principal coral reef harbors belong- 
ing to the United States. 

Many of the harbors from New York City to the Rio Grande 
are protected by sand reefs. The sand shifted by currents along 
shore tends to close the entrances, so that it is necessary to keep 
dredging them. There is an almost continuous inner passage be- 
hind these reefs from Cape Cod Bay to northern Florida. 

Island Harbors. — Islands help to form the harbors of Boston, 
Bombay, and Hong Kong. 



432 PHYSIOGRAPHY 

Harbor Improvements. — Artificial harbors are building at both 
ends of theTanama Canal, and on the Pacific side of the Isthmus 
of Tehuantepec. A well protected harbor has been created at 
La Plata, near Buenos Aires, by building great breakwaters in 
the open roadstead of the La Plata River. 

At New York City deeper channels have been dredged, new 
docks and piers built at right angles to the shore lines, and old 
ones lengthened. 

Destruction of Harbors. — Rivers silt up shallow coastal plain 
harbors, and currents along shore close harbor entrances. Man- 
grove and other forms of plant life, and corals and other forms of 
animal life, fill or obstruct them. 

Her climate is such that for usefulness practically every Rus- 
sian port is destroyed by ice during the winter; and the desire 
for an ice-free harbor has been one great motive for the Russian 
advances toward Constantinople, Persia, India, and the Pacific 
Ocean. 

The destruction of the harbors of an enemy is one of the most 
effective war measures. Alexander the Great destroyed the har- 
bor of Tyre; the Romans that of Carthage; and the Dutch long 
kept closed the entrance to Antwerp. During the Napoleonic 
wars, England endeavored to blockade the ports of France and 
all her allies. 

QUESTIONS 

i. Obtain from Washington, D. C, the last annual report of the 
Lighthouse Board and state the number of men, lights, ships, and 
amount of money used in this way to lessen the dangers that lurk along 
our shores. Compare the number of lights along two different coasts, 
New Jersey and Maine for example, or Puget's Sound and California. 

2. Obtain and study some one Pilot Chart, as for example, that of 
New York City — U. S. Coast and Geodetic Survey Chart No. 121 — 
and note the numerous soundings off shore, the buoys along the chan- 
nels, and the arrangement of the lighthouses to enable vessels to enter 
the harbor after dark. The channel outside Sandy Hook, etc. 

3. Give reasons for believing that the shorelines migrate and state 
what may cause this migration. 

4. Assign to the different members of the class the examination of 



SHORE LINES AND HARBORS 433 

large scale maps of the coasts of the principal states and nations and 
have individual reports made by them in class, with reproductions on 
the blackboard or on manila paper of some of the most characteristic. 

5. In the same way assign the principal harbors and ports of the 
world for careful study in the light of climate, land forms, and shore- 
lines. This work, which is in a sense library laboratory work, makes an 
excellent review and conclusion of the subject. 

6. Compare in a table the effects of (1) elevation, (2) depression and 
(3) the action of waves, currents and tides at shoreline. 

7. Account for branched appearance of Chesapeake Bay, the even 
shoreline of Peru, the deltas of the Mediterranean Sea, the tide ascend- 
ing the Hudson to Albany. 

8. By what means can one locate former shorelines of extinct lakes, 
such as Bonneville and Passaic? 



APPENDIX 

Physiography Reference Library for All High Schools 

Bartholomew: Meteorological Atlas. J. B. Lippincott Co. $10.50. 
Atlas of Commerce. Ginn & Co. $8.00. 

Brigham: Geographical Influences in American History. Ginn & Co. 
$1.25. 

Chamberlain and Salisbury: Geology. Holt & Co. 3 vols. $12.00. 

Davis: Elementary Meteorology. Ginn & Co. $2.50. 
Geographical Essays. Ginn & Co. 

Diller: Educational Series of Rocks. U. S. Geological Survey. (Free.) 

Dryer: Studies in Indiana Geography. Inland Publishing Co., Terre 
Haute. $1.25. 

Gregory, Keller and Bishop: Physical and Commercial Geography. 
Ginn & Co. $3.00. 

Halligan: Fundamentals of Agriculture. D. C. Heath & Co. $1.20. 

Hickson: Story of Life in the Sea. Appleton & Co. 35 cents. 

Hildebrandsson, etc.: International Cloud Atlas. Villars et Fils. 55 
Quai des Grands-Augustus, Paris. (13 francs.) $2.52. 

Laboratory Work: Chamberlain, Darling, Davis, Emerson, Everly, Gil- 
bert and Brigham, New York State Handbook, No. 26, Simmons 
and Richardson, Trafton. 

Mill: International Geography. Appleton & Co. $3.50. 

Moore: Meteorology. Appleton & Co. $3.50. 

Robinson: Commercial Geography. Rand, McNally & Co. $1.25. 

Romanes: Scientific Evidences of Organic Evolution. Macmillan Co. 
50 cents. 

Salisbury: Topographic Maps. Paper. No. 60, U. S. Geological Sur- 
vey. (Free.) 

Semple: American History and Its Geographic Conditions. Houghton, 
Mifflin Co. $3.00. 

Smithsonian Institution: Washington, D. C. Send for catalogue of free 
pamphlets. 

State Geologist : Address for lists of maps and publications. 

Teaching: See Bibliography of Science Teaching. Bulletin 446, U. S. 
Bureau of Education. 

Todd: New Astronomy. American Book Co. $1.30. 



436 PHYSIOGRAPHY 

United States: Washington, D. C. Ask for catalogues, information, 
etc.: 

Agricultural Department. Lighthouse Board. 

Coast and Geodetic Survey. Post Office Department. 

Forest Service. Public Land Office. 

Geological Survey. Weather Bureau. 

Ward: Climatology. Putnams. $2.00. 

Additional Books on Physiography for City or Large School Library 

Avebury , Lord : (Sir John Lubbock.) Scenery of England. Macmillan 
Co. $2.50. 

Scenery of Switzerland. Dyrssen and Pfeifter, N. Y. $1.00. 
Baedeker: United States, Great Britain, etc. Scribner, N. Y. 
Ball: Ice Age. Appleton & Co. $1.50. 
Bonney: Volcanoes. Putnams. $2.00. 
Cowhane: Graphic Lessons in Geography. Westminster School Book 

Depot, London, S.W. 
Croll: Climate and Time. Appleton & Co. $2.50. 
Crosby: Common Minerals and Rocks. D. C. Heath & Co. 60 cents. 
Dana: Manual of Geology. American Book Co. $5.00. 

Manual of Mineralogy. Wiley, N. Y. $1.50. 
Dodge: Readings in Physical Geography. Longmans. 70 cents. 
Geike: Scenery of Scotland. Macmillan Co. $3.25. 
Heilprin: Mt. Pelee. J. B. Lippincott Co. $3.00. 
Hobbs: Earthquakes. Appleton & Co. $2.00. 
Huxley: Physiography. Macmillan Co. $1.80. 
Hogarth: Nearer East. Appleton & Co. $2.00. 
Mackinder: British Isles. Appleton & Co. $2.00. 
Marr: Study of Scenery. New Amsterdam Co., N. Y. $1.50. 
New Jersey Geological Report: Vol. V, Glacial Geology. 
Partsch: Central Europe. Appleton & Co. $2.00. 
Perry: Spinning Tops. Young, London. $1.00. 
Rotch : Sounding the Ocean of Air. Young. $1.00. 
Russell: Lakes of North America. Ginn & Co. $1.50. 

Rivers of North America. Putnams. $2.00. 

Volcanoes of North America. Macmillan Co. $4.00. 
Shaler: Sea and Land. Scribner. $2.50. 

First Book in Geology. D. C. Heath & Co. 60 cents. 

Nature and Man. Scribner. $1.50. 
Suess: Face of the Earth. Oxford University Press, N. Y. 4 vols. 

$28.00. 
Tarr: Physical Geography of New York State. Macmillan Co. $3.50. 



APPENDIX 437 

Tyndall: Forms of Water. Appleton & Co. $1.50. 

Hours of Exercise. Appleton & Co. $2.00. 
Wallace: Distribution of Animal and Plant Life. Humboldt. 15 cents. 

Island Life. Macmillan Co. $1.75. 

Tropical Nature. Macmillan Co. $1.75. 
Winchell: World Life. Scott. $2.50. 
Wright : Ice Age in North America. Appleton & Co. $5.00. 

Equipment in Physiography 

The most important equipment is a well-trained, up-to-date teacher. 
He will know how to select from the following lists, and how to utilize 
and to supplement the local, State, and National equipments. 

1. Maps. — Targe hall maps of continents. These should show depth 

of the sea. (Habenich-Sydon are good.) 
*Public Land Office Map of United States. (Free or $1.25.) 
*State Geological, County, and Railroad Maps. 
*County or City Maps. 

*Grouped United States Topographic Maps of your neighborhood. 
Mississippi River Commission. (Address Secretary, St. Louis, 

Mo.) 
United States Geological Survey — Physiographic Folios 1 (q.s.) 

and 2 (q.s.). Chicago and New York Special Folios. 
Coast and Geodetic Survey Charts, *i2o, *i2i, n, 21, 30, 123, 
143, 145, 154, 204, 314, 337, 359, 408, 419, 469, 1007, 5500, 5532. 
Hydrographic Office: * Pilot Charts of various oceans. 

2. Globes. — Joscelyn 18-inch globe is one of the best. 

*i 2-inch slated globe. 
* 6-inch globe ($.25), q.s. 

3. Models. — *Harvard Geographical Models, (Mountains and Coasts). 

Consult Edwin E. Howell, 612 17th St., Washington, D. C. 
Ward's Natural Science Establishment, Rochester, N. Y. 
Knott Apparatus Co., Boston, Mass. 
Central Scientific Co., 14 Michigan St., Chicago. 

4. Pictures and Lantern Sldjes. — Detroit Photographic Co., Detroit. 

American Bureau of Geography, Winona, Minn. 
W. H. Rau, Philadelphia. 
T. H. McAllister, 49 Nassau St., New York. 
Chicago Geographical Society, Chicago. 
*Department of Visual Instruction, Capitol, Albany, N. Y. 
E. Steiger & Co., New York. 

•Very valuable. 

q. s. — Quantity sufficient to supply every member of a class. 



438 PHYSIOGRAPHY 

5. Minerals, Rocks and Fossils. — Foote Mineral Co., 1317 Arch St., 

Philadelphia. 
Ward's Natural Science Establishment, Rochester, N. Y. 
Central Scientific Co., Chicago. 

6. Instruments. — Standard Thermometer. 

Maximum and Minimum Thermometer. 
*Thermograph. 

Mercurial Barometer. 

Aneroid Barometer. 
*Barograph. 

Hygrometer. 
*Hygrodeik. 

Hygrograph. 

Rain Gauge. 

Wind Vane (with connection with class room). 

Anemometer. 

B-K Solar Indicator. 
*Solar Compasses. 
*Protractors (6-inch zylonite to Y2 arc is very convenient) . 

•Very valuable. 



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